Port to GTK 4: II

Continuation of !156 (closed).

TODO

• Debug errors bellow
• More testing
• Check if the interval var passed in !187 (4879ebe1) is correct
• Cleanup the first commit
• Do a CSS cleanup, many classes are unused or imitate classes that are now in libadwaita
• Maybe restore --enable-experimental-non-null see #214
• App does not close on exit (Seems to be intentional)

On startup there are errors

(org.gnome.clocksDevel:2): GLib-CRITICAL **: 15:41:32.678: '' is not a valid GVariant format string

(org.gnome.clocksDevel:2): GLib-CRITICAL **: 15:41:32.678: g_variant_new_va: assertion 'valid_format_string (format_string, !endptr, NULL)' failed