Extensibility: id/ref

[qbalukom]

Support assigning ids to widgets and referencing them in other widgets. Consider the format:

1. string - namespace/name/id eg. ref: kube-public/qqqs/foo (could fall back to just id if within the same extension)
2. object: eg.

ref:
  namespace: kube-public
  name: qqqs
  id: foo

3. look into the possibility of adapting the existing $ref feature of json-schema (this would replace the currently included ReferencingHandler which isn't really used - except for the extension edit form, but that one has it's own config)