As outlined in #299, we broke many places in the code that were expecting
_album_for_id and _track_for_id to return a single item rather than a list. I
took this opportunity to divide up the interface: there's now one function for
MBIDs (returning a single object or None) and one for generic IDs (returning a
list).
It's possible that these plugins might require a little more thought
into how they should work (or not work) with potentially multiple albums
matching an idea, and potentially those those albums being an incorrect
match (e.g. if two data source plugins share a simple numerical ID
format).
An earlier change (due to @pedros) added the ability for plugins to define
template fields that work with Albums as well as Items. This enables some
cool new use cases but required that every template field definition check the
type of its arguments. Instead, this iteration on the idea distinguishes
between fields meant for Items and those meant for Albums.
In addition to simplifying the implementation of these functions, this also
enables the creation of album fields with identical names to item fields.
(For example, a user contacted me recently about adding a $bitrate field for
albums, which would be the average bitrate of the items. They can do this now
using a plugin.)
I also changed the docs to stop using the decorator approach to registering
template fields. We're moving toward removing those.
