Migrate to GTK4
I did port pavucontrol to Gtk4. I tried to stay as close as possible to the Gtk3 version for the initial proposition, but if people are interested, there are a bunch of changes I would like to add :
- Some refactoring, to avoid code duplication for the popover menus and avoid using ->reference() and ->unreference() directly
- Adding libadwaita as a dependency and using a AdwApplication instead of GtkApplication. This has two advantages: first it fixes a bug in the default dark GtkTheme (I will also write a bug report in Gtk's gitlab so hopefully that bug will be fixed upstream too), and second it makes pavucontrol follow the "Dark mode" option from Gnome-Shell
- Allow renaming devices directly from inside the popover menu instead of using a dedicated popin
- Replace GtkProgressBar by GtkLevelBar for volume to allow things like changing color of the bar depending on volume level