Display player errors and prepare for client websockets (first attempt)

[Neraste]

This PR prepares for the use of websockets on the client and has numerous side effects:

• Digest data are stored in different parts of the state (the state is digest-agnostic);
• Playlist entries in digest have an estimated date of play computed by the client;
• Played and queuing playlist entries can be accessed in a library fashion (using pagination);
• Player errors can be fully accessed in a library fashion (initial aim of the PR, fix Add view to display playlist errors #111).

[Neraste]

This PR started to diverge too much since #138 (I think), it's becoming impossible to resolve the conflicts. I'm considering to re-do this PR again.

[Neraste]

Closed in favor of #156.