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
