evdev: Manage LIBINPUT_EVENT_TOUCH_* events

Those are translated into CLUTTER_TOUCH_* ClutterEvents. As the
"NULL" ClutterEventSequence is special cased, the slot=0 value is
avoided.

Frame events are ignored, as there is no Clutter equivalence, and
Cancel events are sent to all current individual touches.

https://bugzilla.gnome.org/show_bug.cgi?id=728968