[RFC] Port to gtk4/libadwaita, redesign

This MR ports postmarketos-welcome-gtk3 to libadwaita, to make it fit in better with GNOME 42 apps. (not so gtk3 now, huh?)

I'd be willing to also change the UI a bit to match GNOME's HIG a bit more closely, as well as potentially incorporate some of the suggestions in #1 (closed), but this is a bit out-of-scope for this MR - right now, the main goal was just to port it without losing/adding any functionality. Decided to go ahead with this now.

(See video in https://gitlab.com/postmarketOS/postmarketos-welcome-gtk3/-/merge_requests/8#note_946484180)

The code compiles on my end, although it does throw a warning (warning: passing argument 1 of 'gtk_application_get_active_window' from incompatible pointer type...) - as far as I'm aware though, this is correct behavior, as AdwApplication inherits from GtkApplication, and doesn't have its own adw_application_get_active_window method or such... either way, this code seems to work fine. Fixed.

Not entirely sure what to do with the repository name, or even with the repository itself - we could keep the gtk3 version for non-GNOME environments, and make the gtk4 version a separate repo, or manage it as another branch, or not bother with this at all and just replace this version...