Behavior API proof of concept PR

[John Hendrikx]

For those still interested, I've updated 
https://github.com/openjdk/jfx/pull/1265 with a replaced ButtonBehavior 
and SpinnerBehavior, using concepts laid out in 
https://gist.github.com/hjohn/c7b1bf9d4a4770b1b3ae854b20fbaa94

Any feedback is welcome, I think it's making progress, and the changes 
required seem quite doable.

The ability for Behaviors to intercept any event from its Control or its 
(named) children makes Behaviors a lot more powerful than earlier 
proposals, allowing for completely new interactions without requiring 
Skin changes.

--John