[x11] Force FocusIn events for already focused windows

Depending on the current focus stealing prevention level, it's possible for kwin to call XSetInputFocus() on a window that already has the input focus. In which case, we won't receive the corresponding FocusIn event and the client will remain inactive from kwin's perspective even though it isn't.

In order to work around this issue, we can move the input focus to the null window. By doing so, it's guaranteed that we're going to receive the matching FocusIn event for the client.

This commit indirectly fixes a bug where fullscreen games are displayed below panels.