display: cancel wayland client touches when the compositor is grabbed

When a compositor grab begins, clients will stop receiving events, so any
ongoing sequence at that time must be cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=733631
events: Do not swallow touch events on windows

Those might eventually trigger a gesture into recognition.

https://bugzilla.gnome.org/show_bug.cgi?id=733631