NowPlayingFragment: Only set rating visibility on init, connection and config.

Setting the rating visibility was happening per track change, now we only
change it on initialization, connection and if the config option changes.
UpdateTrackInfo: Only attempt rating retrival if the config option is set.
dmix: Final cleanup to try to fix null responses.
MPDStatusMonitor: Don't wait() when connected.

In certain cases, an idle subsystem update event can be skipped due to a wait()
used to keep from looping (and subsequently eating too much CPU) too often
during disconnection. Now, we only only wait() when disconnected.

This is a stop gap fix until this class is rewritten.
MPDApplication: Suppress exception if a disconnection timer is cancelled.

We received an automated crash report that an exception caused a crash due to a
disconnection timer cancellation. This is not a problem as this is part of the
normal course of operations. Log and suppress this exception.