[Scenario] Include fields for duration, start, and end in Info View

Completes implementation of nasa/MCT-Plugins#9, which specifies that info views for activities and decisions should include Start Time, End Time, and Duration fields which could be edited. 

Previously this was not feasible, as those three fields were not independent (duration = end - start). While changes were made to the component model to ensure this relationship was satisfied, it was not reflected in the InfoView: Changing one property did not cause others to be refreshed, nor was there any mechanism for a plugin (such as scenario) to introduce this behavior.

nasa/mct#153 addressed this shortcoming of the platform. This pull request introduces the desired behavior of nasa/MCT-Plugins#9 now that it is possible. Additionally, some refactoring of relevant property editors has been performed for purposes of simplification (consolidating the behavior of the editors for all six fields - three each for decision and activity - into one class)