clutter/x11: Implement keycode lookup from keysyms on virtual key devices
Unfortunately XKeysymToKeycode() falls short in that it coalesces keysyms into keycodes pertaining to the first level (i.e. lowercase). Add a ClutterKeymapX11 method (much alike its GdkKeymap counterpart) to look up all matches for the given keysym.
Two other helper methods have been added so the virtual device can fetch the current keyboard group, and latch modifiers for key emission. Combining all this, the virtual device is now able to handle keycodes in further levels.
Closes: gnome-shell#135 (closed)