[Scenario] Follow up on known issues

Addresses:

* nasa/MCT-Plugins#24 by fixing error in comparison used for formatting (> used where >= should have been used)
* nasa/MCT-Plugins#26 by disallowing changes in duration which would shrink an activity to smaller than a few pixels
* nasa/MCT-Plugins#37 by visiting collections/non-activities while constructing timelines
* nasa/MCT-Plugins#40 by tweaking drawing code

Additionally, drawing code in general has been reviewed/modified to match wireframe diagrams more closely. This also helps nasa/MCT-Plugins#25 (short-duration activities don't look quite as strange with the new drawing code, wherein activities are shorter and more rounded)