When true, the interface will automatically start playing the playlist when (and only when) it is ready. (particularly useful for embedded vouts).
* src/libvlc.c, src/vlc.c: new VLC_AddIntf() prototype.
* src/interface/interface.c: if the interface doesn't support "playing on start", do it ourselves.
* modules/gui/wxwindows/wxwindows.cpp: implement "play on start".