CoverArtDelegate - drawing rows before doing the coverpixmap stuff
CoverArtDelegate - using previous cover_delay to lock/unlock the coverdelegate::paint()

If the CoverDelegate is locked, it must not try to load and search covers.
It means that in this cases it will just draw covers which are already in the pixmapcache.
CoverArtDelegate - extending same logic used to delay cover_loadings when the user is scrolling fast
Cropped Image - storing cropped images in pixmapcache + cropping them in the working threads (search and load images)

For more details, see new commented lines...