clutter/x11: Implement keycode remap to keysyms on virtual key devices
Keycode lookup can fail for serveral reasons, e.g. if there is no combination of modifiers and keycodes that can produce the target keysym with the current keyboard layout.
In case the keycode lookup fails, remap temporarily the keysym to an unused keycodes.
Fixes: gnome-shell#109 (closed)