grabHelper: consume the press/motion/release sequence if a press dismisses the grab

The grab would previously just consume the button release, while propagating
motion events, possibly down to clients in wayland. This would produce
inconsistent streams there.

On pointer events, the inconsistency would just be having clients receiving
events with the button 1 set in the mask, with no implicit grab. When touch
events are handled, this would be more hindering as the client would receive
touch_motion events with no prior touch_down nor later touch_up, something
never supposed to happen.

https://bugzilla.gnome.org/show_bug.cgi?id=733633
grabHelper: handle touch events during grab modality

The "pointer emulating" touch sequence will be handled in order to dismiss
the grab, while the others are just propagated.

https://bugzilla.gnome.org/show_bug.cgi?id=733633