Fix coverart drop indication.

Disable drop if there is no selected item, and if the scheme is not
supported.
Update links to help pages.
Merge pull request #227 from Sophist-UK/sophist_update_help_pages

Update links to help pages.
Merge pull request #216 from Typz/covertArtDropIndication

Fix coverart drop indication.
Merge remote-tracking branch 'upstream/master' into sophist_fix_delete_embeded_images