wayland: Add Xwayland grab keyboard support

This protocol is limited to Xwayland only and is not visible/usable by
any other client.

Mutter uses the following mechanisms to determine if an X11 client
should be granted a grab:

 - is "xwayland-allow-grabs" set?
 - if set, is the client blacklisted?
 - otherwise, has the client set the X11 window property
   _XWAYLAND_MAY_GRAB_KEYBOARD on the window using a client message?
 - if not, is it a client white-listed either via the default system
   list or the settings "xwayland-grab-access-rules"?

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