macosx: fix and improve window level handling

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.