Add grab support for QtWayland::Keyboard

Adds the possibility to grab the wl_keyboard interface, similar to
pointer and touch grabs. Also includes basic unit tests

Change-Id: I665df2ab5adfe16abd6b55f2d00cd9e6dc89d039
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Reviewed-by: Mikko Levonmaa <mikko.levonmaa@lge.com>