feat: Add freedesktop colorscheme preference support

Use DBus to listen to system color scheme changes and update Gajim's GUI
accordingly if dark style 'System' is selected in preferences.

For details about the freedesktop color scheme preference see
https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/
4 jobs for system_style in 3 minutes and 24 seconds (queued for 1 second)