search for dark or light mode stylesheet

summary:
-------
This commit adds xdg-desktop-portal support to waybar. If a portal
supporting `org.freedesktop.portal.Settings` exists, then it will be
queried for the current colorscheme. This colorscheme will then be used
to prefer a `style-light.css` or `style-dark.css` over the basic
`style.css`.

technical details:
-----------------
Appearance is provided by several libraries, such as libhandy (mobile)
and libadwaita. However, waybar links to neither of these libraries. As
the amount of code required to communicate with xdg-desktop portal as a
client is rather minimal, I believe doing so is better than linking to
an additional library.

The Gio library for communicating with dbus is rather messy, Instead of
the `Portal` class containing a `Gio::Dbus::Proxy`, it extends it which
simplifies signal handling.

`Portal` then exposes its own signal, which can be listened to by waybar
to update CSS.

For a reference implementation, please see another one of my projects:
https://github.com/4e554c4c/darkman.nvim/blob/main/portal.go

test plan:
---------
If no desktop portal which provides `Settings` exists, then waybar
continues with the log line
```
[2023-09-06 14:14:37.754] [info] Unable to receive desktop appearance: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.Settings” on object at path /org/freedesktop/portal/desktop
```

Furthermore, if `style-light.css` or `style-dark.css` do not exist, then
`style.css` will still be searched for.

Waybar has been tested with both light and dark startup. E.g. if the
appearance is dark on startup the log lines
```
[2023-09-06 14:27:45.379] [info] Discovered appearance 'dark'
[2023-09-06 14:27:45.379] [debug] Try expanding: $XDG_CONFIG_HOME/waybar/style-dark.css
[2023-09-06 14:27:45.379] [debug] Found config file: $XDG_CONFIG_HOME/waybar/style-dark.css
[2023-09-06 14:27:45.379] [info] Using CSS file /home/pounce/.config/waybar/style-dark.css
```
will be observed.
If the color then changes to light during the operation of waybar, it
will change css files:
```
[2023-09-06 14:28:17.173] [info] Received new appearance 'dark'
[2023-09-06 14:28:17.173] [debug] Try expanding: $XDG_CONFIG_HOME/waybar/style-light.css
[2023-09-06 14:28:17.173] [debug] Found config file: $XDG_CONFIG_HOME/waybar/style-light.css
[2023-09-06 14:28:17.173] [info] Using CSS file /home/pounce/.config/waybar/style-light.css
```

Finally, tested resetting waybar and toggling style (works, and style is
only changed once).

fixes: Alexays/Waybar#1973

src/util/portal.cpp

@@ -0,0 +1,106 @@
+#include "util/portal.hpp"
+
+#include <giomm/dbusproxy.h>
+#include <glibmm/variant.h>
+#include <spdlog/spdlog.h>
+
+#include <iostream>
+#include <string>
+
+#include "fmt/format.h"
+
+namespace waybar {
+static constexpr const char* PORTAL_BUS_NAME = "org.freedesktop.portal.Desktop";
+static constexpr const char* PORTAL_OBJ_PATH = "/org/freedesktop/portal/desktop";
+static constexpr const char* PORTAL_INTERFACE = "org.freedesktop.portal.Settings";
+static constexpr const char* PORTAL_NAMESPACE = "org.freedesktop.appearance";
+static constexpr const char* PORTAL_KEY = "color-scheme";
+}  // namespace waybar
+
+using namespace Gio;
+
+auto fmt::formatter<waybar::Appearance>::format(waybar::Appearance c, format_context& ctx) const {
+  string_view name;
+  switch (c) {
+    case waybar::Appearance::LIGHT:
+      name = "light";
+      break;
+    case waybar::Appearance::DARK:
+      name = "dark";
+      break;
+    default:
+      name = "unknown";
+      break;
+  }
+  return formatter<string_view>::format(name, ctx);
+}
+
+waybar::Portal::Portal()
+    : DBus::Proxy(DBus::Connection::get_sync(DBus::BusType::BUS_TYPE_SESSION), PORTAL_BUS_NAME,
+                  PORTAL_OBJ_PATH, PORTAL_INTERFACE),
+      currentMode(Appearance::UNKNOWN) {
+  refreshAppearance();
+};
+
+void waybar::Portal::refreshAppearance() {
+  auto params = Glib::Variant<std::tuple<Glib::ustring, Glib::ustring>>::create(
+      {PORTAL_NAMESPACE, PORTAL_KEY});
+  Glib::VariantBase response;
+  try {
+    response = call_sync(std::string(PORTAL_INTERFACE) + ".Read", params);
+  } catch (const Glib::Error& e) {
+    spdlog::info("Unable to receive desktop appearance: {}", std::string(e.what()));
+    return;
+  }
+
+  // unfortunately, the response is triple-nested, with type (v<v<uint32_t>>),
+  // so we have cast thrice. This is a variation from the freedesktop standard
+  // (it should only be doubly nested) but all implementations appear to do so.
+  //
+  // xdg-desktop-portal 1.17 will fix this issue with a new `ReadOne` method,
+  // but this version is not yet released.
+  // TODO(xdg-desktop-portal v1.17): switch to ReadOne
+  auto container = Glib::VariantBase::cast_dynamic<Glib::VariantContainerBase>(response);
+  Glib::VariantBase modev;
+  container.get_child(modev, 0);
+  auto mode =
+      Glib::VariantBase::cast_dynamic<Glib::Variant<Glib::Variant<Glib::Variant<uint32_t>>>>(modev)
+          .get()
+          .get()
+          .get();
+  auto newMode = Appearance(mode);
+  if (newMode == currentMode) {
+    return;
+  }
+  spdlog::info("Discovered appearance '{}'", newMode);
+  currentMode = newMode;
+  m_signal_appearance_changed.emit(currentMode);
+}
+
+waybar::Appearance waybar::Portal::getAppearance() { return currentMode; };
+
+void waybar::Portal::on_signal(const Glib::ustring& sender_name, const Glib::ustring& signal_name,
+                               const Glib::VariantContainerBase& parameters) {
+  spdlog::debug("Received signal {}", (std::string)signal_name);
+  if (signal_name != "SettingChanged" || parameters.get_n_children() != 3) {
+    return;
+  }
+  Glib::VariantBase nspcv, keyv, valuev;
+  parameters.get_child(nspcv, 0);
+  parameters.get_child(keyv, 1);
+  parameters.get_child(valuev, 2);
+  auto nspc = Glib::VariantBase::cast_dynamic<Glib::Variant<std::string>>(nspcv).get();
+  auto key = Glib::VariantBase::cast_dynamic<Glib::Variant<std::string>>(keyv).get();
+  if (nspc != PORTAL_NAMESPACE || key != PORTAL_KEY) {
+    return;
+  }
+  auto value =
+      Glib::VariantBase::cast_dynamic<Glib::Variant<Glib::Variant<uint32_t>>>(valuev).get().get();
+  auto newMode = Appearance(value);
+  if (newMode == currentMode) {
+    return;
+  }
+  spdlog::info("Received new appearance '{}'", newMode);
+  currentMode = newMode;
+  m_signal_appearance_changed.emit(currentMode);
+}