Providing Node.localToWindow()

I started playing with SubScene a few days ago.
And I realized that I could not use Node.localToScene() as I did before.

When there is no SubScene in the scene graph, Node.localToScene() is
basically a way to convert local coordinates to window coordinates (because
Scene and Window can be considered as one entity).

When there are SubScene's, then Node.localToScene() convert local
coordinates to coordinates *relative to the closest enclosing SubScene*.
These coordinates are not relative to the window anymore. To get window
coordinates, you need to call Node.localToScene() up to the root Scene.

This computation could be performed by FX and named Node.localToWindow()
for instance. With SubScene now in the picture, this seems pretty consistent
with existing FX design.