backend-x11: Ensure the Xkb group index remains properly set

Ubuntu ships a patch in the X server that makes the group switch
keybindings only work on key release, i.e. the X server internal group
locking happens on key release which means that mutter gets the
XKB_KEY_ISO_Next_Group key press event, does its XLockGroup() call
with a new index and then, on key release, the X server moves the
index further again.

We can work around this without affecting our behavior in unpatched X
servers by doing a XLockGroup() every time we're notified of the
locked group changing if it doesn't match what we requested.

https://bugzilla.gnome.org/show_bug.cgi?id=756543