Comment 16 for bug 801700

I don't think it is necessary to unlock the mutex manually or put them in an extra scope in the context I'm using them in.

In the new version addTracksPrepare checks if there are unfinished transactions and closes them if they are found and I overloaded getTrackValueForColumn. The new getTrackValueForColumn now gets as additional parameter a reference to the QVariant. I left the old on also in because other functions in BTC also use it and changing them to the new getTrackValueForColumn results in empty rows in the library (I'm not sure why).