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/client.cpp

@@ -151,8 +151,26 @@ void waybar::Client::handleDeferredMonitorRemoval(Glib::RefPtr<Gdk::Monitor> mon
   outputs_.remove_if([&monitor](const auto &output) { return output.monitor == monitor; });
 }
 
-const std::string waybar::Client::getStyle(const std::string &style) {
-  auto css_file = style.empty() ? Config::findConfigPath({"style.css"}) : style;
+const std::string waybar::Client::getStyle(const std::string &style,
+                                           std::optional<Appearance> appearance = std::nullopt) {
+  std::optional<std::string> css_file;
+  if (!style.empty()) {
+    css_file = style;
+  } else {
+    std::vector<std::string> search_files;
+    switch (appearance.value_or(portal->getAppearance())) {
+      case waybar::Appearance::LIGHT:
+        search_files.push_back("style-light.css");
+        break;
+      case waybar::Appearance::DARK:
+        search_files.push_back("style-dark.css");
+        break;
+      case waybar::Appearance::UNKNOWN:
+        break;
+    }
+    search_files.push_back("style.css");
+    css_file = Config::findConfigPath(search_files);
+  }
   if (!css_file) {
     throw std::runtime_error("Missing required resource files");
   }
@@ -235,8 +253,15 @@ int waybar::Client::main(int argc, char *argv[]) {
   }
   wl_display = gdk_wayland_display_get_wl_display(gdk_display->gobj());
   config.load(config_opt);
+  if (!portal) {
+    portal = std::make_unique<waybar::Portal>();
+  }
   auto css_file = getStyle(style_opt);
   setupCss(css_file);
+  portal->signal_appearance_changed().connect([&](waybar::Appearance appearance) {
+    auto css_file = getStyle(style_opt, appearance);
+    setupCss(css_file);
+  });
   bindInterfaces();
   gtk_app->hold();
   gtk_app->run();
@@ -244,4 +269,8 @@ int waybar::Client::main(int argc, char *argv[]) {
   return 0;
 }
 
-void waybar::Client::reset() { gtk_app->quit(); }
+void waybar::Client::reset() {
+  gtk_app->quit();
+  // delete signal handler for css changes
+  portal->signal_appearance_changed().clear();
+}