keybindings: Mask out the reserved modifiers mask
When switching layouts, special modifiers bits may be be set for internal use by Xkb.
As we now ignore a set of modifiers when processing the special modifiers keys, we ought to also mask out those reserved modifiers otherwise we would discard the [Super] key after switching layouts in X11.
Closes: #1144 (closed)
!1219 (merged)
Edited by Olivier Fourdan