Allow undoing making a node into a field

[taramk]

Should be a more elegant option than just deleting it. We could either concatenate the key + value with a space between, or we could just use the value for the node text.

[progrium]

If you've gone to the point of setting a value, I don't really understand why/when somebody would want to go back to a normal node.

[taramk]

I'm thinking mostly of cases where they've made a mistake (so they likely haven't set a value).

[progrium]

How many individual solutions to one-off undo should we have before figuring out undo?