[Scenario] Add Scenario component & view

This resolves nasa/MCT-Plugins#13 

A new Scenario component is added which serves as a container for multiple Timeline objects

A corresponding Scenario view is added which displays multiple collapsible Scenario views on the same time scale, along with cost graphs (both per-Timeline, and one big one for the whole set of timelines). Note that embedded Timeline views still expose Timeline Inspectors in this context.

Additionally, Activity views within a timeline are colorized according to their activity Type, which may mitigate the effect of nasa/MCT-Plugins#25