Advanced fetchart source config: add validation function
Advanced fetchart source config: add validation function

ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,SusannaMaria/beets,ibmibmibm/beets,beetbox/beets,sampsyo/beets,shamangeorge/beets,shamangeorge/beets,shamangeorge/beets,sampsyo/beets,SusannaMaria/beets,sampsyo/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,jackwilsdon/beets,ibmibmibm/beets,beetbox/beets,shamangeorge/beets,SusannaMaria/beets,jackwilsdon/beets,SusannaMaria/beets,beetbox/beets