Add Id property to Category class.
Fix outport names and messages.

We're not actually outputting a builtincategory object so we shouldn't
be showing that on the port. I override it here so I can still have the
behavior of other enum drop down types.
Merge remote-tracking branch 'origin/master' into Issue2401
Add tests for category node.
Merge pull request #2645 from DynamoDS/Issue2401

Issue2401