Fix #327 : List cover art progress bar spins forever
- Cover progress bar display is now triggered by the message "onCoverDownloadStarted" sent by the cover manager.
- The cover is then displayed by the album cover listener when it receives the onCoverDownloadStarted message.
- The cover manager sends the onCoverDownloadStarted message when the cover is downloaded from the web (no progress bar when the cover comes from the cache)