DUI: key navigation. Part 2

Yellow navigation is from [Part 1](https://github.com/Benglin/Dynamo/pull/123).
Pink is navigation from this PR.
![image](https://cloud.githubusercontent.com/assets/8158404/4661657/85a9c2c0-5526-11e4-824d-334693b8f66f.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)
3. Press up/down to move from member group to class button. (inside one category)
4. Press left/right to navigate between class buttons. (inside one category)

Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5073](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5073) DUI: Navigate between Classes
```cs
// No class is found in this 'SearchCategory', return from here so that higher level
// element gets to handle the navigational keys to move focus to the previous category.
if (searchCategoryContent.Classes.Count == 0)
    return;
```