Allow to select children even if parent is disabled

[bosquig]

Hi,

I have a use case where I don't handle my root elements like the other nodes, and I don't want the user to be able to select the root elements and put it in the value (its not even the same entity in my backend).

So I need to disable the root elements, but the problem is that now I couldn't select the other nodes (except if flat = true but that doesn't fit my need).

I figured a way to do what I wanted with this pull request, and I guess it can be somewhat useful to some as it's not limited to root elements. I don't know though if it breaks something elsewhere but I didn't see anything broken in my tests and it would break only if the new node prop were to be set to true anyway.

Hope this makes sense.

[hngl]

I am looking for this exact feature: disabled parent while keeping child selectable.

@riophae Would you consider merging this?