Adding Copy/Paste/Cut options for AnimationTree nodes

[jcarlosrc]

Describe the project you are working on:
A 3d third person game.

Describe the problem or limitation you are having in your project:
I have to manage multiple animations and re-make the AnimationTree system some times to find the best way. For instance I want to move a BlendSpace2d node into another blending node. However, there is no way to move a node into another without re-making it which is time consuming. It would be good some simple options such as copy, paste, cut, duplicate and similar.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Adding the options when right mouse input is pressed and enable keys such as Ctrl + C, ...

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

If this enhancement will not be used often, can it be worked around with a few lines of script?:
It is a core and useful feature for general purpose.

Is there a reason why this should be core and not an add-on in the asset library?:

[Calinou]

Duplicate of #955 and #106.