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
I think ```e.Handled``` is always ```false``` when it comes into this method, can we move this all the way up to the top of this method so it looks like this? Let's see if that works for you:

```cs
e.Handled = false;
if ((e.Key != Key.Down) && (e.Key != Key.Up))
    return;
```