DUI: Object type check should be made more robust

The following problem is from https://github.com/Benglin/Dynamo/pull/60 this pull request, it should be fixed:

There is a foreach loop in `ArrangeOverride` method of `LibraryWrapPanel` class. It checks to see if a given child is `StandardPanel` or a regular class icon. The way it checks is by going through the width of the child object, this is not a reliable way of checking.

Instead, the check should be done by checking the runtime object type 
(I think it is StandardPanel object). <-- That didn't work, but check of `DataContext` did.
just for fun... i needed to try :cat: 

Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4682 Object type check should be made more robust](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4682)

That's strange, if the ```child``` is not ```StandardPanel```, is it a parent of ```StandardPanel``` or what is it?