DUI: key navigation(use is selected). Part 1

EB wants to have focus in search text box and key navigation together.
That's why we will use `IsSelected` property instead of `Focus()`.

What's done:
* In `LibraryContainerView` on preview key down added one more case: for `Key.Down` and `Key.Up`. Here key down event generates programmatically.
* `HighlightedItem` is currently selected item. For this item KeyDown is generating now.
* `OnMembersListBoxKeyDown` helps to navigate inside members of MemberGroup.
* Everything was renamed.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5395](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5395) DUI: Navigate by IsSelected instead of Focus()