Progress Bar Performance

A couple very quick changes for the progress bar, specifically avoiding walking back slowly, and one fix for some fairly common lengthy itemset re-evaluations.
In the case of an ItemSet, the return from the widget has to be restricted by the available options. The available options are cached at the FormEntryPrompt level, but when we navigate through the form we create a new prompt each time, since FormEntryPrompt is a dynamic form element (For example, the itemset might need to refer to new stuff it things change), so this just makes sure we're keeping around that cached set of questions when we go look to the answer.