[Scenario] Revise time editing in Info View

For activities in the Info View, keep
duration constant and shift other times
as needed when adjusting start or end
time, as requested in nasa/MCT-Plugins#110.

Additionally, avoid validating that
start is less than end, as the shifting
behavior means this is no longer
relevant (and refusing to do so makes
it hard to move an Activity farther
into the future.)