cosmetic fix - fixing style
CoverCache - saving defaultCover for tracks which do not have cover

In this way Mixxx will run the search algorithm just once.
It avoids a heavy search loop - which can easily leads to performance issues
CoverDelegate - removing outdated comments
TrackTable never displays the default cover + CoverCache::requestPixmap() updates CoverInfo when coverLoc and md5 are already in the DBHash
CoverCache - storing QImage(defaultCover) as a member
WCoverArt - removing unused member
WCoverArt - storing defaultCover pixmap as a member
cosmetic fix - wcoverart - getting CoverCache instance only once.
WCoverArtLabel - removing unused member