DUI: key navigation.Part 6

This PR should be merged after [that one](https://github.com/Benglin/Dynamo/pull/147) to escape conflicts.
This PR makes possible creation of nodes after ENTER button was pressed.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5195](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5195) DUI: On Enter for member a node should be created
[MAGN-4851](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4851) DUI: Top result should be highlighted by default (no tooltip) and should add the node when pressing enter
This method should have been named as ```ExecuteSearchElement```, because the outcome of clicking on a search item will not always result in creation of a node, it depends on the ```Execute``` method.