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.
tim - fixing, thx.

muellmusik - you're right. It creates problems like this.