solve issue #2370 (deeply clone MotionEvent path)

[stephengold]

Currently a work in progress.

[stephengold]

This PR was created to address issue #2370. Here's what it does:

• It adds code to MotionEvent.cloneFields() to clone the path field. While implementing this change, I discovered that the lookAt and rotation fields also needed to be cloned. (The direction and upVector fields are cloned in the jmeClone() method, which is irregular but suffices for most use cases.)

• It makes MotionPath and Spline deeply cloneable, adding jmeClone() and cloneFields() methods. Unfortunately, Cloner.clone() does not work on ArrayList<Float> because Float is not cloneable---see issue Cloner fails on enums and wrapped primitives #879. In Spline.cloneFields() I used copy constructors to work around this limitation.

• It adds automated cloning tests for all 3 classes to jme3-core/src/test. The cloning tests for Spline use the same test splines as the existing serialization tests. To avoid duplicating code, I refactored the generation of test splines to private methods.

• The copyright notices in the affected source files were updated to include the year 2025.

[yaRnMcDonuts]

Since no one else has reviewed this yet, I did my best to look through the code and I don't see any apparent issues.

So I will leave this open for another day or two and then I plan to merge this in ~48 hours.