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
After understanding your comments and the overall XAML layout, I would recommend rewriting the comment as follow:

```cs
// The 'StackPanel' that contains 'SubCategoryListView' and 'MemberGroupsListBox'
// handles this message. If none of these two list boxes are handling the key 
// message, that means the currently focused list box item is the item in these two
// list boxes. When key message arrives here, it is then the 'StackPanel' job to 
// move the focus on to the adjacent list box.
```