Implemented skinnable theme songs.
Plays a theme song when opsu! starts (can be disabled in options).  The default song is "welcome to osu!" by nekodex, uploaded by CyberKitsune.

To change the song, place a new "theme.osu" in the skins folder and edit the file name, metadata, and song length (at minimum).

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>