allow to read metadata of currently played track from mediaplayer

allow to read metadata of currently played track from mediaplayer

basically gives external code access to the information the media player
provides in his gui, namely artist, album, title, genre, year plus the
path to the coverart (unless there is none).
you can use e.g. the filestreamer to retrieve the coverart afterwards.

please note that this does not use any official api of the mediaplayer
but rather accesses the labels+pixmap directly.