has_front -> want_front, has_back -> want_back

Those are defined according to user config option for accepted caa types.
Introduce caa_has_front and caa_has_back, reduce code redundancy
has_caa_artwork -> caa_has_suitable_artwork

It reflects much more its purpose as even if caa has artwork we may have this set to False
Differentiate case of no CAA info vs no cover art