Add documentation for AbstractWindow

[jwiggins]

Closes #626

[aaronayres35]

LGTM

Just one request:
could we change # to #: in the trait defs for the AbstractWindow class so they are also picked up by the api docs, not just the methods

[jwiggins]

could we change # to #: in the trait defs for the AbstractWindow class so they are also picked up by the api docs

I actually prefer for them not to show up. Can you think of any traits that should be exposed to user code?

[aaronayres35]

could we change # to #: in the trait defs for the AbstractWindow class so they are also picked up by the api docs

I actually prefer for them not to show up. Can you think of any traits that should be exposed to user code?

I guess that makes sense. It seems most (all?) the traits will be dealt with by enable inner machinery.

I think I am still a bit unclear on when we anticipate a user needing to mess with the AbstractWindow. Is the custom cursor scenario pretty much the only one that is common?
We are putting this in the user manual but I get the vibe that we don't really want users to touch or have to think about it

[jwiggins]

Thanks for the feedback