Use CoverInfo also in dlgtrackinfo

I missed to clean up some slots to use the new CoverInfo struct.
REF dlgtrackinfo only use signals to get covers.

Do not get the cover directly from the coverartcache in dlgtrackinfo
instead rely on the answer from the signal. The signal is guaranteed to
return a pixmap even if that is the default pixmap.
remove unused m_firstCoverLocation
Don't use hidden expensive `fieldIndex` function in cover delegate

the `trackmodel->getTrackID` function is using `fieldIndex` itself. This
can cause some performance issues on low hardware so we go the same way
here as everywhere else. I also remove some unnecessary class members.
Destroy CoverArtCache in MixxxMainWindow

The class that creates a singelton should also be responsible to destroy
it. I also moved the comment to tell people when it has to be destroyed.
Merge branch 'coverArt-fixes' of https://github.com/kain88-de/mixxx into coverArtSupport
CoverDAO cleanup - calling deleteUnusedCoverArts() directly