Pass all key events during Alt+Tab to the Wayland Seat

When Alt+(Shift)+Tab is grabbed we have the modifier press key being
passed to the Wayland server as key events are not yet grabbed. As KWin
grabbed the release event the Wayland server considered the key still as
pressed when going into the next application.

This change ensures that all events are also passed to the wayland
server. As no surface has focus we can be sure that it won't be passed
to the application, but it ensures that the events are processed
correctly.

Reviewed-By: bshah