Fix: Double-clicking on Python node does not bring up editor

This is meant to fix the following user reported issue:

[No more double-click to edit Python scripts?](https://github.com/ikeough/Dynamo/issues/677)

Internal defect: [MAGN-528 Double click on python nodes does not open for edit](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-528)

It appears the second click in a double-click event is eaten by **dynNodeView.topControl_MouseLeftButtonDown** method, but it does not actually do anything with it. Now we only set **e.Handled** to **true** if the method actually handles the click event. This way the second click has the chance of getting to **dynPython.nodeUI_MouseDown** method for processing (instead of eating it up).