DUI: key navigation. Part 1

This PR is about navigation between members in member groups.
Where we can navigate:
- up to member;
- down to member;
- up to member group;
- down to member group.

Just to remind:
![image](https://cloud.githubusercontent.com/assets/8158404/4629967/69d7005e-53a9-11e4-8f73-b6b83a199966.png)

How to reproduce:
1. Click on any member. (This click will give it focus)
2. Press up/down button to navigate through members and member groups. (inside one category)

Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4851](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4851) DUI: Add arrow keys navigation for selecting each of the items under lists

Please rewrite the above statements (sorry, I am obsessed about the code being readable):

```cs
var itemIndex = 0;
if (e.Key == Key.Up)
    itemIndex = nextMemberGroup.Items.Count - 1;

var generator = nextMemberGroup.ItemContainerGenerator;
(generator.ContainerFromIndex(itemIndex) as ListBoxItem).Focus();
```