Migrate to gtk4

Rebased and continued version of !173 (closed).

cc @feliwir @bberg @kyteinsky @BrainBlasted @felipeborges

In order to make review as straightforward as possible, the original commits were rebased with further changes made by myself added on top and kept separately for now. Before merging this, my plan is to squash all of these commits into a single one, with @feliwir named as author and myself as co-author. I'm open to handling this differently, though.

Changes compared to the original MR:

• libadwaita is referenced via pkg-config like other deps, instead of bundling it as a subproject
• the addition of icons for the sinks was reverted as the chromecast icon didn't seem to work
• review comments on the original MR were addressed. I'd especially appreciate feedback on lorbus/gnome-network-displays@0702dcd4
• Some unrelated compilation warnings were fixed too.
• complete the move to gtk4

There are still some issues with this, even if I personally wouldn't block this MR on them being resolved:

• min window height is too large when width is small, becomes smaller as width gets larger:
• error view is not styled properly:
• default size has (unneeded) scroll bar:

Supersedes: !173 (closed)

Fixes: #293 (closed)