MPDStatusMonitor: When updating, the status is changing, not the database.

According to MPD Protocol Reference, Chapter 3, the server will emit an update
idle response to an update command. This does not mean the database has
actually changed, only that the server is checking for updates, making this
an actual status change.
MPDStatusMonitor: Let the user know if the database actually changed.

Let the user know if the database actually changed. This will only happen after
the first parameter of libraryStateChange has changed to true to false.