applets/kickoff: allow removing favorites through drag&drop

Favorites can be added through drag&drop now, but not removed this way.

This change adds this; dropping a favorite from its gridview on the sidebar or footer will cause it to be unfavorited. The exception is the sidebar entry for the favorites page; dropping things there is used to add favorites, so removing them with the same action seems semantically inconsistent.

---

This currently does not have a removal placeholder; kickoff does not have one for adding to favorites either (unlike kicker(dash)). Instead it just hides the delegate when it would be removed. We could add a removal placeholder later if we add an insertion placeholder (and a way to switch to the favorites page mid-drag to make it useful).