Allow disabling nodes

[RichardJECooke]

Currently, you can click the node's eye in the editor, but that only hides the mesh. It doesn't prevent processing, or stop the invisible mesh hiding shadow on other objects. We need the eye to actually disable the node like it's been taken out of the tree.

[arkology]

Like this?

[RichardJECooke]

Yes, but 1) in the editor (not editor or code as the issue gives as an optio)n and 2) it should be called 'enabled' or 'disabled' as this is the standard name in programming languages, not 'stashed'.

[Calinou]

Thanks for the proposal! Consolidating in #7715.