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Pointers for TWMPDClient; and MPDWatcher tells client when things change.

I think it's cool. Where we previously manually sent a message down the
pipe saying "playlist changed" when someone added a song to the
playlist, now the MPDWatcher sends a message whenever any subsystem
changes, and what subsystem changed.

This is useful in that it now tells clients whenever anything changes,
like playlist, player, other things...

Basically, it *should* catch deletes now.