Prioritized event handlers

[Michael Strauß]

Hi Andy!

In a previous mail, I observed that a method like
`BehaviorBase.addHandlerLast` suggests to me that we're lacking core
tools in the JavaFX framework. In this case, we want to ensure that an
event handler is always invoked before (or after) other event
handlers.

One way to do that might be to support prioritized event handlers at
the core level, instead of creating new API that seems like a
workaround. I've created a proof of concept for prioritized event
handlers [0], which might be a moderate-effort but high-value addition
to JavaFX.

[0] https://github.com/openjdk/jfx/pull/1266