gdk: Turn GdkEvent into a GObject

gdk: Turn GdkEvent into a GObject

Two warts remain. gdk_event_copy() should be unnecessary as
events should be considered static after delivery, so g_object_ref()
should be just as good. There's a few exceptional cases that the event
is copied and then modifier for later processing, those cases should be
reconsidered individually.

And gdk_event_free() could be likewise turned into g_object_unref(),
many callers remain though.