TrackDAO - add new signal to updateTracksInBTC
- It is doing the same thing that the signal "tracksAdded" does.
- However, it makes the code more readable (considering the purpose)...
CoverCache - set maximum cover size from 400px to 300px
CoverCache - rescaling covers before calcule md5 hash
- md5 calculation might be a very slow process for big covers
Fixing bug in searchImage() - the md5Hash string must start with the value got previously in the db
-  it will be essential to identify if Mixxx is just searching for an embedded cover that was previously loaded. Avoiding recalculate the md5 hash...
cosmetic fix - fixing commented line