gtk: apply all window appearance changes in syncAppearance (#5404)

The GTK side of appearance code is kind of a mess with several different
functions all having the responsibility of interacting with each other
and setting the appropriate window appearance. It should solely be the
responsibility of the `syncAppearance` function to apply appearance
changes, with other callbacks/functions calling it instead: much like
what we already do for the macOS apprt.

~~I also took the time to refactor the libadwaita version checks since
calling `versionAtLeast(0, 0, 0)` does get old after a while. Now almost
all checks are given human-readable names and contributors need not
memorize what the relevant version checks all are.~~ Moved to another PR