Display and cache album art from any source in the artist's album listing

In the artist's album view, load cover art using the `CoverAsyncWorker`, instead of the custom code that was there.

This means we can load from the LAN or LastFM and cache artwork directly in that listing - instead of only in the album song listing.

Notes:

- You still have to have caching switched on to get any artwork loaded.
- The cached will be checked first to see if an image is present, before continuing to load from first LAN and then LastFM (builds on 2027f54b).
- I also check isWifi before downloading anything, so as not to overload a 3G connection

@abarisain I considered another config option to enable this feature, but I didn't as I know you don't want too many! The final item in the notes above was added to provide a sensible default, since there's no config option to switch this feature on.