actions: Add VoidAction(), mirroring NoSymbol/VoidSymbol. Added `VoidAction()` action to match the keysym pair `NoSymbol` / `VoidSymbol`. It enables overriding a previous action and breaks latches. This is a libxkbcommon extension. When serializing it will be converted to `LockControls(controls=none,affect=neither)` for backward compatibility. We cannot serialize it to `NoAction()`, as it would be dropped in e.g. the context of multiple actions.