gtkgesture: Apply gesture group state after setting up sequence
When a new sequence is added to a GtkGesture, its state is looked in other gestures in the same group, and made to match in this gesture. This however happened a bit too early, before the gesture touchpoint was fully set up. As this may result in signal emission and whatnot, it's a good idea to make it happen with a fully set up touchpoint.
Fixes: #3946 (closed)