gdbus: Initialize types earlier to break proxy <-> connection deadlock
gdbus: Initialize types earlier to break proxy <-> connection deadlock

This will help us break generic GType deadlocks between people using
GDBus in different threads (which is supported), not just by GType
usage in the GDBus thread.

This should fix the common cases we're seeing in the wild, although I
have some lingering concerns that if someone e.g. referenced
e.g. `G_TYPE_DBUS_AUTH_MECHANISM_SHA1` etc. we'd need to add those
too.

https://bugzilla.gnome.org/show_bug.cgi?id=674885

johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib