Add code to handle NSDiffableDataSource

[gcasa]

The test for this is in https://github.com/gcasa/NSDiffableDataSource_test.git

The test on the mac:

[gcasa]

Ready for review.

[gcasa]

I notice lots of enumerator based while loops. Should we expose the FOR_IN macro from GSFastEnumeration.h in the base library, so that GUI code can use fast enumeration irrespective of what compiler is in use?

Not only would itrun those loops faster, it would also make the code shorter and more readable/maintainable.

I have replaced every instance of this with FOR_IN(...).

[gcasa]

@fredkiefer, let me know what else I need to change. I believe I have addressed everything. Any other issues can be addressed in a separate PR.

[gcasa]

@fredkiefer Are you okay with merging this?

[fredkiefer]

I still must admit that I did only understand less than half the code in this PR. It is way too big for me to properly review. But if you yourself feel comfortable with the changes, go ahead and merge them.

[gcasa]

@fredkiefer Merging, per your comment.

[gcasa]

I will work to reduce the size of PRs moving forward.