<fb-playlist-tabs> user dragged a tab to a new position.
<fb-playlist-tabs>
Original index.
The new order as indices into the playlists the tabs listed.
The same order as playlist GUIDs, the form sent to fb.playlist.reorderPlaylists.
fb.playlist.reorderPlaylists
Drop-target slot index (already adjusted for drag direction).
<fb-playlist-tabs>user dragged a tab to a new position.