dmix: Code style.
CommandQueue: If separated server command response is empty, insert empty list.

In the past we have discarded empty results. This can lead to confusion for why
a list went missing when it's expected in the results. With this commit we now
add a empty list when there is nothing between the list separator responses.
MPDApplication: Don't crash when dialog not dismissed, application terminates.
MPDApplication: Don't call mpd.disconnect() in the disconnect callback.

We've already disconnected!
MPDConnection: Process available commands only with a valid header.
MPDStatusMonitor: Update queue/status/statistics before connection callback.
MPDControl: Add more media keys.