[RFC] Switch to GTK4/libadwaita, redesign

(Currently set as "draft" since I'm waiting for !3 (merged) to land so that I can rebase this and add the "don't show again" switch.)

Before:

After:

Two notable changes:

• The upgrade message was moved directly into the .ui file.
• The link to "reporting issues" was removed; this is better off on the wiki page IMO.