Add a way to control volume of public speaker.

[dag10]

Allow DJ-Listener to be launched with a argument designating it as a public speaker, and will therefore let the server change its output volume. Then, authenticated users (or perhaps just admins) will be able to view and modify the volume of public speakers connected to the room.

[dag10]

Along a similar vein, have a speaker icon show up next to anonymous listeners count indicating if a public speaker is connected. Perhaps this is where its volume can be controlled.

This speaker icon can also be shown on the homepage/roomlist to indicate that a room is connected to a public speaker. This speaker icon should be the silhouette of a bookshelf speaker to avoid confusion with the mute button.