pan-action: emit ::pan-stopped before ::gesture-begin

When starting a new gesture before the momentum of the previous one
has finished the ::pan-stopped was counter-intuitively emitted
after the new ::gesture-begin.

Make use of gesture_prepare() to reset the state of the action
right before emitting ::gesture-begin.

https://bugzilla.gnome.org/show_bug.cgi?id=683431