Feature: support playlist edits.

[2e0byo]

Currently there is no support for moving items in playlists, inserting, or bulk actions.

From a comment on this pr:

a first look at the call made by the Tidal client shows a POST request to the playlist item URL with the toIndex argument

looks promising. I wonder if toIndex would let additions work as insertions? In any case it would be nice to support:

• moving tracks
• inserting at given index
• bulk actions (if the backend api supports it)

These (particularly the first two) would be very handy in mopidy-tidal.

I guess it's a matter of spinning up MITMProxy again...

[tehkillerbee]

@2e0byo FYI, some of this functionality is already implemented in tidal2: https://github.com/arnesongit/plugin.audio.tidal2