backgroundMenu: Release the click gesture if a grab operation begun

backgroundMenu: Release the click gesture if a grab operation begun

If an active grab on pointer events happens during multi-touch operations,
all non-pointer-emulating touches will be muted. This may leave the
Clutter.ClickAction incomplete if triggered by one of those sequences,
just to have a gesture take over and trigger a compositor grab, which would
leave the capture-event handler stuck eating events.

So listen for grab-op-begin from the display, and ensure the action is
released if such grab begins.
and the capture event handler stuck.