`exit_sketch()` method doesn't work correctly for JavaFX

[hx2A]

See discussion thread:

Originally posted by @vsquared in #664 (comment)

This probably requires an updated exitActual() method in SketchBase to better support JavaFX, based on the code in @vsquared's comment.

Once this is fixed we can fix #656

[hx2A]

Useful comment:

#664 (reply in thread)

How to stop the animation thread? This should be added to exitActual().