Don't push an empty playlist folder on the scanner's queue
Under certain circumstances the scanner wouldn't pick up new artwork,
even if music file was changed.

Wipe the artwork scanner's cache once the scan has finished. Otherwise
stale results might be returned if the same album is scanned again soon
after.
Merge remote-tracking branch 'origin/public/7.7' into public/10.0
7.8 changes are not part of 10.0. Remove changelog for 7.8.