x11: update inhibit shortcuts on grab broken

On X11, shortcuts inhibition is emulated using a grab on the keyboard.

So if another widget ungrabs the keyboard behind our back (for example when a popup window is dismissed) that effectively disables the effects of the shortcut inhibition on the surface and we need to update the shortcut inhibition status accordingly.

Check for "grab-broken" events on the surface and clear existing shortcuts inhibition for the matching seat, so that the client can be notified and may decide to re-enable shortcut inhibition if desired.