[rfc/fix] woa -no progress dialog when start playing from main menu

ref http://trac.xbmc.org/ticket/14954

playback is started in background-thread therefore woa does not give visual feedback (progress-dialog).

suggested solution ; access file from gui-thread (using File::Exists()) to have woa activated before spawning playback-thread.
I don have a deep overview of the playback system (very little, in fact;-), so ;
- is there any danger or drawback in doing it where I suggest in this PR?
- are there other similar places where some of the same scenario can pop up?