added: musicplayer.position(number).*, musicplayer.offset(number).* info labels and...

added: musicplayer.position(number).*, musicplayer.offset(number).* info labels and boolean conditions for retrieving items from the music nowplaying list. offset is relative to the current playing item, position is relative to the start of the playlist.  We also have conditions musicplayer.hasnext and musicplayer.hasprevious as well as musicplayer.offset(number).exists to check whether playlist items exist.  Thanks to kraqh3d for the original patch.

git-svn-id: https://xbmc.svn.sourceforge.net/svnroot/xbmc/trunk/XBMC@13315 568bbfeb-2a22-0410-94d2-cc84cf5bfa90