Bump GTK+ and GLib minimum versions to 3.22 and 2.52

Several Xfce components already require GTK+ 3.22, and we'll need it when we bump GtkSourceView to 4.0. GTK+ 3.22 only requires GLib 2.49.4, but I'd like to push it a little bit higher, to be able to use g_utf8_make_valid() (we currently use a copy in mousepad/mousepad-util.c).

This would avoid adding more compile switches, as I did recently in !60 (merged) (for GTK+ 3.22), and as I would have to do again to address #4 (closed) (for GLib 2.46).