Add gnome-mplayer to list of supported audio players

May I ask someone more knowledgeable why the sound applet operates with a whitelist for working media players rather than a blacklist for those that do not?  As a noobie looking in from the outside (a.k.a. me), it seems like an unnecessary burden to need to add these players one-by-one.  If I am not mistaken, unity uses a blacklist already, for example.

Addresses issue #1618

Requires gnome-mplayer 1.0.7 so I have not tested this (the default repo is still at 1.0.6).