xwayland: add _XWAYLAND_MAY_GRAB_KEYBOARD property

Add a new client message "_XWAYLAND_MAY_GRAB_KEYBOARD" that X11 clients
can use to tell mutter this is a well behaving X11 client so it may
grant the keyboard grabs when requested.

An X11 client wishing to be granted Xwayland grabs by gnome-shell/mutter
must send a ClientMessage to the root window with:

 - message_type set to "_XWAYLAND_MAY_GRAB_KEYBOARD"
 - window set to the xid of the window on which the grab is to be issued
 - data.l[0] to a non-zero value

Note: Sending this client message when running a plain native X11
environment would have no effect.

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