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.
I don't think we want ```Engine``` to be dependent on ```PresentationCore```. The idea is that ```Engine``` is a language runtime and it does not understand anything about visuals. Please remove all these new references that you have added. The annotation (e.g. tool-tip description or icon image) of a library item should be a ```UI-only``` thing, that is done outside of ```Engine```. Typically how this happens is having either an external XML file that describe how each class/method is associated with the icon, and tool-tip description text; or we can have another assembly alongside the original assembly that supplies the annotation information.