ISIS-478: starting work on dependencies between action parameters.

* ToDoItem now has category and subcategory properties
  - disabled
  - ToDoItem#updateCategory action now updates both
* introducing subscriber logic between ActionParametersFormPanel and the ScalarPanelAbstract (subclasses)

also:
- deleted ProcessObjectPanelAbstract, not formal API and not used
- deleted DropDownChoicesForXxx, superceded a while back by the Select2Choices
ISIS-478: further work to getting choices to be updated between action parameters

* completed subscriber logic between ActionParametersFormPanel and the ScalarPanelAbstracts
* ObjectActionParameterContributee subtype's getChoices() now takes argumentsIfAvailable
  * used when invoke facet
ISIS-478: fixing quickstart example app tests
ISIS-478: dependent choices now refreshed in UI ok.
ISIS-478: choices and default methods can now take partial parameter lists

See ToDoItemContributions#updateCost and its supporting methods for example.