ISIS-210: choices with param types now also supported.

* changes to ObjectActionParameterContributee subtypes and ActionParameterChoicesFacet to support choices taking parameter types
* refactored to simplify impl of defaults also

in addition:
* updated ToDo example, factored out ToDoItemContributions to demonstrate usage with a new "updateCategory" action
* combined ToDoItemsJdo with ToDoItems service
* new installAndReturnFirst() prototype action

also:
* fixed minor issue with wicket dropdown; the current value must be in the list of choices, else will NPE, so adds automatically
* enhanced rendering of actions to show @DescribedAs as a title