Allow the addReplace action to replace existing nodes while keeping the ...

...same node id.

This allows transparent swapping of a synth (e.g. updating synthdefs with a new version) while ensuring all references to the node remain valid. Allow this on Groups for consistancy, though it's functionally identical to freeing all of the Group's children.
Re white space: I think it might be time to standardise on one style. Whitespace inconsistencies are a very common issue with submitted patches, but derive to a large extent from the fact that massive numbers of files were added that did not follow the original conventions.

I know even mentioning tabs vs. spaces risks starting a holy war, but honestly we should just clean this up.