ISIS-953: enhancements to FixtureScript.ExecutionContext

Also:
- refactoring the todoapp and simpleapp to use FixtureScripts, lookup(...), and using composite fixtures.
ISIS-928: refactoring unit tests of simpleapp to use nested static class format.
ISIS-928: recreating simpleapp archetype
ISIS-928: recreating todoapp archetype
ISIS-928: further tiny tweaks to todoapp example & archetype.
ISIS-537: fixing the hints for entity page, plus trivial tidy-up.

Specifically:
- removing unused parameter to CollectionSelectorPanel's constructor
- removing comments.