DUI: key navigation(use is selected).Part 2

This PR should be merged after [that one](https://github.com/Benglin/Dynamo/pull/193).
Here tooltip appearce + Enter creates node.

Reviewers
@Benglin ,@vmoyseenko, please take a look.
This looks odd, but I'm sure [the task about removing HighlightedItem](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5409) will take care of this.