Improve player behavior when items are moved from / to the end of a playlist

[defagos]

As a user I want a consistent playlist user experience when moving items around. Currently the behavior once the playlist is exhausted is not very consistent.

Acceptance criteria

The following questions have been answered:

• How should the player behave after the last item ended but a previous item is then moved after it (or another item is appended)?
• Same question if the last item, whose end has been reached, is moved elsewhere in the playlist.

Tasks

• [ ] Document answer to these questions.
• [ ] Create stories if needed.