Autocomplete function signatures - Part 3

This is part 3 of the bigger submission: https://github.com/DynamoDS/Dynamo/pull/2688.

Added function overload insight window for function signature tooltip. This submission adds new classes (inheriting from AvalonEdit) to display function signature UI/tooltip.

@Benglin pls review.
This event should not be required, I have created [another internal task](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5106) to track the removal of it.