CoverDAO::getCoverInfo() - improving performance by removing "indexOf()" calls

The method getCoverInfo() can be called hundreds or even thousands times (correctly - it will depends on the library size) mainly in cases when the user is scrolling fast in a new library.

I noticed that it were doing many "indexOf" calls in order to get the column numbers and as each "indexOf" call will do a new loop and this was resulting in a  considerable loss of performance... just to do unnecessary things...
WTrackTableView - getting coverLocation and md5 column numbers just once
- After some analysis with valgrind, I noticed that fieldIndex() can takes a long time...
- (see commented lines)