Fix for Node Preview size

Fixes an issue with the Node Preview, where the first time the preview is generated, it's a small default size instead of the size it should be.  This is evident on long preview boxes the first time you hover over them.

You actually want to Measure the TextBox, a child of smallContentGrid, to get it's size and make the preview that size instead.  Add a 10 units for a margin.  I'm looping through all children, but that might not be necessary.

Related to Issue 1930, but I can't see how to link the issue with the pull request.  #gitnoob
https://github.com/DynamoDS/Dynamo/issues/1930
Please could you factor this "10" into Dynamo.UI.Configurations.PreviewMargin or something similar? :)