types: fix various use of volatile in type registration
!! This is still waiting on the following MR from GLib to land !!
The use of volatile was incorrect in GLib and has been that way for a long time. Recently however that has changed, and this makes GTK follow suit to avoid using volatile in the type registration.
See also: glib!1719 (merged)
Combined with the above merge request for GLib, this fixes a large number of compilation warnings when using Clang.