Deprecate the search path for ~/.themes in GTK3/4, remove in GTK5
~/.themes
is a legacy path and should no longer be supported. It might be a problem to remove the search path in the middle of GTK4 (and backport patches to GTK2 and GTK3), so let's deprecate them and remove them when the time for GTK5 comes.
Required to remove the search path for ~/.themes
in KDE: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/3585#note_816672