applets/kickoff: allow adding favorites through drag&drop

Kickoff only allows adding favorites through the context menu.

This change also allows it by dragging items from the list/grid views to the favorites sidebar entry. Dropping there will add it to the end of the current favorites list, just like using the right-click menu does.

In the future, a delayed hover-activation could be added to allow immediately putting the new favorite in a particular position; but we likely want the favorites sidebar delegate to be droppable anyway, in case the user plans to add multiple things from one category.