Focus Traversal API for JDK 9

[Jonathan Giles]

Hi folks.

I'm continuing along with working on a few small API additions for JDK 9 
to smooth off some of the rough edges due to Jigsaw modularity. Today 
there is a relatively big API to discuss - focus traversal.

In summary, my plan is to essentially bring forward the focus traversal 
API out of com.sun.javafx.scene.traversal and move it to 
javafx.scene.traversal. I have done that on my machine, and along with a 
few cleanups to reduce the API surface area and to clarify some generic 
class names, it is ready for review. This isn't to say it is final, or 
even guaranteed to go in to JDK 9, but I wanted to get your thoughts on 
it so that we can move forward with the javadoc effort, etc. The JavaDoc 
that you see presently is unchanged from what was there previously, so a 
major task still to do is to fully document the new API.

If it proves contentious, the likely plan would be to return to this 
again after JavaOne, and if things can't be resolved it might not make 
it into JDK 9.

I've uploaded the latest javadocs for this API to my website, which you 
can see here:
http://jonathangiles.net/javafx/jdk9/traversal/

Any thoughts you have would be welcome!
Thanks,
-- Jonathan