Ability to retrieve a `Node` from a `JNode`

[acasteigts]

It might be useful to be able to retrieve the Node corresponding to a given JNode. For instance, this happened in an attempt to detect where a node was dropped after being dragged. To this effect, I suggest to add a method getNode() in class JNode.

[acasteigts]

What's been done in branch feature/99-node-from-jnode:

• Added a getNode() method in class JNode

Merged into develop.

[acasteigts]

(Will be closed when released.)

[remikey]

Looks good to me. Ready to be shipped in next release.

Rmq: I don't think this new piece of code is used anywhere in the production/test/example code of the project. This is not vital, but we should try and add something in that direction, should the occasion arise.