Improving CoverArtDelegate::paint() - remove unnecessary QStyleOptionViewItem copy
Improving CoverArtDelegate::paint() - using trackmodel as a member to get it just once
Improving CoverArtDelegate::paint() - using defaultCoverLocation as a member to get it just once from CoverCache
Improving CoverArtDelegate::paint()

- getting coverLocation and md5 column just once;
- ordering calls in a better way;
After some analysis with valgrind, I noticed that fieldIndex() takes a long time and that we could call it just once (in the construction).
CoverArtCache - checking if the cover found is different from the cover that is already stored in db
- it avoids calling 'updates' when there is nothing to update...
CoverArtDAO - add method to save multi cover arts at once (from qlist)
TrackDAO - updating multi cover_art rows at once (needs more work)
- it should be done in a single query...
CoverArtCache - accumulate covers for half second and then update the database at once

- sqlite can only do about 50 writes per second, so it is important to collect all new covers and write them at once.
TrackDAO::updateCoverArt() - updating multiple coverIds in a single query
TrackDAO::updateCoverArt() - trackIds and coverIds must have the same size
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)...

 we should create a new signal with a better name, updateTracksInBTC?