[Scenario] Validate times entered via info view

Ensure that start, end, and duration times entered
for an Activity or Decision via the Info View are
valid with respect to other times set for that
object. Specifically, disallow start times that
happen after end times, and disallow any negative
times (times less than zero).
Merge pull request #74 from VWoeltjen/validation

[Scenario] Validate times entered via info view