Fix MAGN-5027 Dynamo doesn't notice changes to levels in Revit

<h4>Summary</h4>
Originally when an element is modified, the related node is not marked as dirty. As a result, when you run the graph once again, the graph will not be run. As a result, the element in Revit is not restored.

This issue exists not only in the Level nodes but also in other nodes as well.

The fix here is to add the event handler to monitor whether elements have been modified. If so, the related nodes will be found out and marked as dirty.

<h4>Test</h4>

It is manually tested and also a test case has been added to track the issue.

@lukechurch 
PTAL