gtkspinbutton: Group up/down button gestures with self
We are adding click gestures on the up/down buttons, but can't let the GtkButton built-in ones prevent ours to run.
As the saying goes, if you can't beat them, join them. Group the spinbutton and GtkButton gestures together, so it's irrelevant which gets called first and ends up winning.
Fixes: #3085 (closed)