Allow PlaylistItem to contain episodes.

[kstep]

Description

It's a change for meta-issue #127: "Allow PlaylistTrack/PlaylistItem to contain episodes."

Motivation and Context

According to meta-issue #127 and official docs, PlaylistItem object can have either FullTrack or FullEpisode object as a value of the track field.
Hence the fix.

Dependencies

Update docs and changelog.

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Run a full test suite.

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This change is 

[kstep]

As a first version, I just changed the PlaylistItem::track field type to Option<PlayingItem>.
It's not the best solution, as it renders the PlayingItem type name imperfect for its function. It should be either renamed (please advise on the better name), or a new type should be introduced. I don't like the introduction of a new type, as it will be just a duplicate of the PlayingItem type, which is not DRY at all.

[marioortizmanero]

Why is PlayingItem's name imperfect? Both a track and an episode are playable types.

[kstep]

Because if the item is part of PlaylistItem, it is not necessary playing at this point in time, as it is implied by the type name ("playing item"), but it's definitely playable. If you think it's OK, then we can go on with this PR.

[marioortizmanero]

Ahh okay. Perhaps just PlayableItem?

[kstep]

Renamed PlayingItem to PlayableItem and updated the changelog.

[marioortizmanero]

Merged and updated the meta-issue.