Cleaning up labels-for-PDGP WIP
Restrict available single labels for PDGPs

[#46492277] The labels available to be associated individually to a PredefinedGraphPane should (a) belong to the current Activity, and (b) not be part of an existing LabelSet (those should be associated as LabelSets). This uses an instance method on the Activity which first collects all GraphLabels descended from that Activity, then discards duplicates and those already associated with LabelSets.