input-device: Do not unset the device stage after the last touch is lifted

On X11 the pointer will follow a "pointer emulating" touch sequence, so the
pointer will be effectively left inside the stage after that touch is lifted,
even though the master device stage is unset. This makes pointer events get
ignored until the pointer leaves and enters again the stage.

https://bugzilla.gnome.org/show_bug.cgi?id=732234
x11: Set the input device stage on XI_TouchBegin, if not already set

Until now, touch events sort of rely on XI_Enter/XI_Leave events accompanying
the pointer emulating touch in order to have a stage set on the device, These
events won't happen though if it's not a pointer emulating touch which happens
on the stage, causing touch events to be ignored.

Fix this by ensuring that the input device has a stage on XI_TouchBegin itself,
but only if it's not already set, so we don't possibly steal touch events to
an already interacting stage.

https://bugzilla.gnome.org/show_bug.cgi?id=732234
gesture-action: Ignore any other event than press/update/release ones

CLUTTER_ENTER/LEAVE might be processed too, leading to accounting of the
NULL sequence (ie. pointer) in the gesture, and fooling the gesture with
a static extra point that wouldn't go away.

https://bugzilla.gnome.org/show_bug.cgi?id=732235
gesture-action: Use event type getter

Don't use direct struct access.
Release Clutter 1.19.4 (snapshot)
Post-release version bump to 1.19.5