Document Batman.BinarySetOperation

Adds docs and tests for `Batman.BinarySetOperation`, `Batman.SetUnion`, `Batman.SetIntersection` and `Batman.SetComplement`. 

Also moves Navigator docs to the "App internals" namespace. 

btw, makes me think that maybe `_itemsWereAddedtoSource`/`_itemsWereRemovedFromSource` could be part of `BinarySetOperation`'s public interface. Doesn't seem like anyone is banging down the door for more Batman.Set operations though ...