clutter: Do not latch modifiers on modifier keys

If the user maps eg. Alt+F2 to a pad button, the MetaInputSettings will send the full Alt press, F2 press, F2 release, Alt release sequence. However the keycode corresponding to Alt is found in level 1, so the Shift modifier gets unintendedly latched in addition to the Alt key press/release pair.

We could probably improve keycode lookup heuristics so level=0 (and no modifier latching) is preferred, but we can do without it altogether for modifier keys.