Convert UpdateManager to a singleton.

## Why
There should only be one UpdateManager in Dynamo.

## How
* The UpdateManager is made a singleton and its required reference to a DynamoModel object is removed.
* All tests for the UpdateManager have been fixed, and their speed has been greatly improved by testing the UpdateManager in isolation. We no longer need to start Dynamo to conduct these tests.
* All UI tests for the UpdateManager are fixed by mocking the singleton. These tests have been removed from the "Failing" category.
* As part of the DynamoModel refactoring and also as part of this pull request, data bindings were broken between the GraphUpdateNotificationControl (the little green cloud), and its data source which should now be the singleton UpdateManager. These data bindings are now fixed.

This pull request addresses internal issue:
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4224

@pboyer PTAL
As long as that external component is available across the application, which the UpdateManager is, I don't have a problem with it. Another way to tackle this would be to make the UpdateManager singleton the data source for the portion of the about window where those versions are compared, then just make the converter do the coloration/formatting based on the comparison of two version numbers.

The situation would be the same if the data context of that portion of the about menu