Adding tracks by referencing other playlist(s)

[EricLambrecht]

As a user I want to be able to add a whole playlist (as a reference) to the currently viewed playlist.

The added tracks should stay grouped together and form one entity (like a track stack in Logic) when dragged & dropped for example. However, it would be useful to "cut" the link to the original playlist (lose the reference) and add the tracks like normal tracks.

Some more ideas:

• The playlist references should behave like a single track, but it would be nice if it can be "unfolded" to show the containing tracks.
• It would also be nice if the playlist reference has an indicator whether or not it is "out of sync" (i.e. if the referenced playlist was edited after referencing it). Another option would be to automatically keep the reference up-to-date (by long-polling etc.).

[EricLambrecht]

This should also be useful when creating new playlists from scratch! (#39)

[EricLambrecht]

Track grouping (#30) could be implemented beforehand, making this feature probably easier.