[MainWindow] Accept content type "Music" as well

MediaObject2 spec has been changed so "audio.music" type is now
"music". We should try to play both types.

Fixes #8. Note that actual fixes need to happen on dleyna-server
side: This just makes dleyna-control accept all legal values of
"Type".

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>