MPD backend doesn't support password authentication

An MPD server may require clients to authenticate via password before executing certain commands. Music Applet supported this, but Panflute does not.

The difficulty here is how to prompt the user for a password, since there may be zero, one, or many clients connected to the Panflute daemon, and the daemon by design has no way to communicate with the user itself. There aren't yet any other instances where the daemon needs to prompt the user for something in order to work, either.

Also, I don't know how commonly MPD password authentication is actually used; not being able to do it may not actually be a concern. The main question is whether this impacts any users of Panflute; I'm inclined to let this go unfixed until I know it does.