DUI: try to remove LibrarySearchView.HighlightedItem

Priority: _Minor_
It was discussed [here](https://github.com/Benglin/Dynamo/pull/193), that it would be great to remove `HighlightedItem` or make it private.
I have created 2 new methods:
- `SelectNext`, that generates KeyDown event for currently highlighted item.
- `UpdateHighlightedItem`, that unselects old item and selects new one.

Also I have read old comments once more and updated them.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5409](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5409) DUI: Try to remove LibrarySearchView.HighlightedItem