Organize API with Mixin Classes

[Erotemic]

This builds on top of my previous PR.

The idea here is start organizing the API to make it easier to keep track of what API calls have a Python call, and which don't. Currently the change is minimal to demonstrate the idea.

Basically for each group of related functionalities the idea is to make a single "mixin" class that contains those methods. Then the API class is a nearly empty class that just inherits from all of the "mixin" classes, thus providing a backwards compatibility.

However, now we don't need comments to delineate grouping of functionality as it is implicit in the code.

As an example of what I'm trying to do, I've had a lot of success with this pattern in my CocoDataset class. I think this pattern will have value here as well.

The current group of mixins uses the same structure implied by the comments. We have:

• InternalAPIMixin - for the protected internal methods
• BiggerAPIMixin - which was just labeled as the "Bigger section of the Jellyfin api". Not sure if there is a better name for that.
• GranularAPIMixin - same deal as above, I just named it based on the comment.
• SyncPlayAPIMixin - This one makes a lot of sense, its functions related to sync play
• API - this inherits from all of the above, which means this PR introduces no user-facing changes. It just makes it easier to organize and continue development.

I also added a doctest to API, which won't run, but illustrates how to create an instance of it. If the testing tools have something that can spin up a dummy server, then we could make this test actually run with xdoctest.

[iwalton3]

This idea seems reasonable enough to me and would improve readability without breaking the API. I say go for it.

[Erotemic]

I rebased on master and dropped other commits not related to this PR. Should be good to review & merge.

[Erotemic]

@iwalton3 If you have any time, merging this would make it easier to me to push up new API features. I've written endpoints for exteranl provider (e.g. IMDB) identification, collection creation, and metadata editing that will have a cleaner diff once this is merged.