XWayland: no forwarding with Shift keys

The shift key doesn't make sense to be one of the keys that will trigger KWin to allow XWayland apps to process all keys when it's pressed, because there aren't generally global shortcuts that consist of Shift plus an alphanumeric character key. That's because this key combination is used to type capital letters.

So we can safely exclude the shift key to improve security against key logging and not break any global shortcut-using XWayland apps for people using this feature.

The shift key on it's own is still forwarded as that's a non-character key.