Make the GtkPlacesSidebar show drop targets, so it can show the available drop targets and a "new bookmark" row. This improves the Drag-and-Drop experience of the user and allows applications to show all available drop targets at once.
This needs to be called when the application is aware of an ongoing drag that might target the sidebar. The drop-targets-visible state will be unset automatically if the drag finishes in the GtkPlacesSidebar. You only need to unset the state when the drag ends on some other widget on your application.