Initial commit for tooltip

Tooltip was made as it assumed to be. As it was expected, tooltip covers Scrollbar and user can not use the scroll with mouse until tooltip is opened. Please review code and close that pull request  https://github.com/DynamoDS/Dynamo/pull/2026 because it was made in the wrong brach. I apologize about that wrong pull request, that will not happen again.
It's better to keep maximum length of lines to be around 80 to 100 characters max. I would suggest the following changes (please consider installing [Visual Studio Editor Guidelines extension](http://visualstudiogallery.msdn.microsoft.com/0fbf2878-e678-4577-9fdb-9030389b338c/) and put guidelines at 80 and 100 characters):

```cs
var format = ```string(/doc/namespaces/namespace[@name='{0}']/category)```;
var obj = XmlDocument.XPathEvaluate(String.Format(format, namespaceName));
return obj.ToString().Trim();
```