Either g_type_register_static_simple (used by G_DEFINE_TYPE_EXTENDED)
and G_IMPLEMENT_INTERFACE use automatic variables for GTypeInfo and
GInterfaceInfo structs, while tutorials and source code often use
static variables. This commit consistently adopts the former method.
https://bugzilla.gnome.org/show_bug.cgi?id=600158
Add dummy for _gdk_input_window_crossing (). Set both input_window_destroy
and input_window_crossing pointers in the Impl struct.
Reported by John Ralls.