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.
This control is highly specific to the Library, perhaps we should call it ```LibraryToolTip```? Also, let's follow [the way WPF names](http://msdn.microsoft.com/en-us/library/ms754034(v=vs.110).aspx) these as ```ToolTip``` and not ```Tooltip``` :)