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: GNOME/gtk#3946

(Cherry-picked from commit 53df32e6)