Make Point2D and Point3D implement Interpolatable

[Nir Lisker]

Hi Kevin/Johan,

I'd like to propose that javafx.geometry.Point2D and Point3D implement
javafx.animation.Interpolatable. Color is the only class that implements it
currently.
As a usage example, this would allow to interpolate over the axis property
of a Rotate or a RotateTransition. In general, interpolating between points
can have many uses in Timeline.

The implementation is trivial - the double coordinates (x and y, and z for
Point3D) are interpolated simultaneously, similar to how Color implements
it.

For your consideration,
Nir