CoverArtCache::changeCoverArt - rescale cover right after its loading
CoverArtCache::requestPixmap - removing unnecessary "requestRepaint" emission

The signal "requestPaint" will be used by widgets which are already calling "requestPixmap" in a paint event.
In this way, emit this signal into "requestPixmap" does not make sense.
Cosmetic fix - rename the parameter "bool fromDelegate" to "bool issueRepaint"
CoverCache - fixing bug introduced by the last changes in the "requestPixmap" parameters
Cosmetic fixes - renaming const vars using lowerCamelCase and starting them with the letter 'k'
Simplifying CoverArtDAO::deleteUnusedCoverArts() - doing everything at once
CoverArtDAO - cosmetic fix - following code convention