[Feature] Loading/Play external audio tracks

This PR adds the possibility to play external audio tracks together with video files and stacked videos.
At the moment the files have to be placed either in the same directory or in a subdirectory named "audio" or "tracks". The files can also be zipped or rared.
The recognised file extensions are read from ```g_settings.m_musicExtensions```.

This closes ticket http://trac.xbmc.org/ticket/11446 .