window: Properly handle focusing override redirect windows

If an app pops up an OR window and sets input focus to it, like
Steam does, we'll think the focus window is null, causing us to
think the app is not focused.

OR windows should not be special if they get input focus, where
the input focus would be set to NULL. Instead, the window should
be marked as focused.

https://bugzilla.gnome.org/show_bug.cgi?id=647706