overrides: Add Gdk.EventTouch union discrimination

Add EventTouch to get/setattr pass-through to support BEGIN, UPDATE, END,
and CANCEL touch event types. Ensure Gdk.Event methods get_state(),
get_axis(), get_coords(), and get_root_coords() are patched onto
Gdk.EventTouch.

https://bugzilla.gnome.org/show_bug.cgi?id=736380