wayland: add inhibit shortcut mechanism

Add a mechanism to MetaWaylandSurface that inhibits compositor's own
shortcuts when the surface has input focus, so that clients can receive
all key events regardless of the compositor own shortcuts.

This will help with implementing "fake" active grabs in Wayland and
XWayland clients.

https://bugzilla.gnome.org/show_bug.cgi?id=783342