Hide slideshow when media_gallery is disabled #2002

Slideshow widget behavior now matches the description.  When media gallery is disabled, the slideshow turns off.

I would love to have been able to disable the save button on the widget creation form, but that's part of os_boxes and it would be hacky to remove.  Also it wouldn't help if someone enabled media gallery, saved a slideshow, and disabled media gallery.
Return array() in case title or other options are added to default box
$block with content is required, my mistake
Merge pull request #2040 from openscholar/2002-slideshow_media_gallery

Hide slideshow when media_gallery is disabled #2002