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/gnome-shell#109
2 jobs for fix-issue-109 in 4 minutes and 14 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #185907
build-mutter

00:01:08

 
  Test
passed #185908
test-mutter

00:03:05