Compare potential cover filenames case-insensitively.
Handle COVER_NONE covers in WCoverArtMenu::slotChange.
Support multiple-selection for cover art menu in WTrackTableView.

* Adds CoverArtCache::guessCover(s) methods for calling
  CoverArtUtils::guessCoverArt in a background thread.
* Removes processing of reset menu option from WCoverArtMenu and instead
  sends a signal that the owner of the menu should handle.
Show cover art progress on the library scanner dialog.
Update BaseTrackCache once cover art processing is done.