DUI: key navigation. Part 4

State: _Critical_
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).
Green is from [third PR](https://github.com/Benglin/Dynamo/pull/134).
Blue is from this one.
This PR makes navigation between categories, top result and search textbox possible.
![image](https://cloud.githubusercontent.com/assets/8158404/4732524/36b5dbf4-59bd-11e4-9b7e-d7da29db80d2.png)
This PR should be merged after [third](https://github.com/Benglin/Dynamo/pull/134).
Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5074](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5074) DUI: Navigation should start from Top Result
```cs
// "MainGrid" contains both "topResultListBox" and "CategoryListView". When 
// "KeyDown" event bubbles up to the level of "MainGrid", it will then decide 
// on which element should the focus go (depending on the navigational key).
// This typically happens during the following scenarios:
// 
// 1. Up/down key is pressed when focus in on "topResultListBox"
// 2. Up key is pressed when focus is on first row of first category
// 
```