macosx: fix and improve window level handling
- fix behavior of video-on-top by adapting to vout windows handling changes - set all windows to status level if one vout window has this level: This avoids that video effects panel, audio efffects panel etc. pp. are opened behind a vout window. Now they can be used as usual. Please note, that due to the type of these panels they do not remain visible when VLC gets inactive.