remove default arg from WCoverArtMenu::show(...)
Remove unused m_pConfig member from WCoverArt.
Revert "don't look up MD5 hash from DB, always save found cover"

This reverts commit 25bd8f77c214f9b5abc5be68119449ccfe79688a.

I'm going to refactor `requestPixmap` as well to use `CoverArtInfo` as
well. `requestPixmap` needs the MD5 hash value.
always calculate MD5 hash for cover found during search
use CoverArtInfo also for `loadImage`
use a CoverInfo struct to request a cover.
use CoverInfo in every class that uses Covers

This makes it easy now to send additional things around by just updating
the CoverInfo struct.
load image when we know they are in the id3tag

with this change the search function is only called if we really don't
know the cover for a track or it cannot be loaded anymore. At all other
times we call the load function. As a nice side effect this reduces the
DB calls for tracks collections where most tracks store the coverart in
the id3tags.
fix broken test
add test for embedded coverart
Fix but to run CoverArtCache alone
fix test and use ID3TAG as location for embedded covers.

We are now stating that a cover is saved as a ID3TAG in the
coverlocation instead of using an empty string. It is a bit more verbose.