ISIS-716: reverting change back to 3.3.6...

... appeared to cause a regression in Estatio with its polymorphic relationship from CommunicationChannel#owner (to either FixedAsset or Party).
ISIS-716: execute "dependent" scripts through the ExecutionContext...
... so can track which scripts have been executed and ensure only ever called once.  This tracking is done by class name.  This allows the idiom that each script can in effect "declare" its dependencies, making it easier to compose and reuse.

Also:
- added a new withTracing() method to print out fixtures that were executed.
- refactored/simplified the quickstart/todo app fixtures to better demonstrate the preferred pattern.