UpdateManager fixes: MAGN-4508

@lukechurch @ikeough 

In order to prevent a crash, Revit shutdown logic now occurs in the Idle Thread.

There was a race condition where the update button would appear in the UI before an update was available. Clicking the button before the update was actually downloaded would cause the update the fail. This was fixed with a pretty dirty patch that duplicates a property originally used for the button binding. This new property is only updated once the download has been completed. This was a less risky fix, since the original property is also tied up in a bunch of other logic within the UpdateManager.