Redo the logic for computing drop positions

Redo the logic for computing drop positions

Instead of having an accept_uri_drops flag, we'll always advertise as accepting URI drops.
However, we'll see if the caller actually handles the drag-action-requested signal.  If it
does, we'll indeed allow performing file operations when URIs are dropped.  Otherwise,
we'll only allow creating bookmarks from dragged URIs.

Dragging URIs directly into a places item will cause the sidebar to check if the caller
allows file operations.  Dragging URIs between places items will cause the sidebar
to create bookmarks for those URIs instead.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>