cocoa_common: fix focus with --ontop and space switching

The main problem this commit addresses is when you switched spaced back and
forth with `--ontop` active the video window didn't recive focus again.

Turns out that setting a normal window level just before conceding focus to
other apps / spaces fixes the problem. I think Cocoa is misbehaving here, and
should probably file a radar.

I found 3 related improvements while fixing this:
 *  fullscreen_window_level is a dead body from an older implementation.
 *  Use NSScreenSaverWindowLevel for ontop. This should be a really high window
    level. Definitely higher than NSNormalWindowLevel + 1.
 *  The window level was set correctly only when out of fullscreen.