DUI: key navigation. Part 3

Yellow navigation is from [first PR](https://github.com/Benglin/Dynamo/pull/123).
Pink is from [second PR](https://github.com/Benglin/Dynamo/pull/127).
And green is from this one.
This PR makes navigation between categories possible.
![image](https://cloud.githubusercontent.com/assets/8158404/4702751/8d194b42-5865-11e4-9709-ef0f2deffccb.png)
This PR should be merged after [second](https://github.com/Benglin/Dynamo/pull/127).

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5072](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5072) DUI: Navigate between Categories
Please see if the following documentation is correct, if so please rephrase the wordings as below:

```cs
/// <summary>
/// 'CategoryListView' element contains the following child elements (either 
/// directly, or indirectly nested): 'StackPanel', 'SubCategoryListView',
/// 'MemberGroupsListBox' and 'MembersListBox'. If none of these child elements 
/// choose to process the key event, it gets bubbled up here. This typically 
/// happens for the following scenarios:
/// 
/// 1. Down key is pressed when selection is on last entry of 'MembersListBox'
/// 2. Up key is pressed when selection is on first item of 'SubCategoryListView'
/// 3. Up key is pressed when selection is on the first entry of 'MembersListBox'
///    and there are no classes.
/// 
/// </summary>
```