Add a option to disable getting artist thumbnails and pictures from Internet!

[CharlieJiang]

Hi all,
I think MusicBee is really an amazing music player but there is a problem is because songs and artist in my library are pretty unpopular so MusicBee always download wrong artist pictures from Internet and some of them are really ridiculous.
So a option to disable getting pictures from Internet does help a lot. Now I can only prevent the problem by disable the writing permission for the artwork folder, which is obviously not a permanent solution.
Thanks!