MAGN-5488 Dynamo can fail to launch when Kiwi Bonus Tools add-in is installed on Revit 2015

This pull request fixes the issue where other installed addins will conflict with Dynamo. That is a symptom which is still unexplained by the Revit team. The root cause of the issue is to do with our attempting to register an event handler for Revit's idle event while handling the idle event. The additional hack to do our Dynamo Revit initialization on idle is not required, as the execute method is, by its very nature, safe for making Revit API calls. This PR moves the logic from OnRevitIdleOnce inside the external command's Execute method.

The issue outlined in MAGN-5488 was reproduced in the Revit 2016 branch without other addins installed. This solution fixed the problem there.

PTAL:
- [x] @Benglin 

Merge:
- [ ] Revit2015