Dui key navigation(from category to category)

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)

Reviewers
@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5072](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5072) DUI: Navigate between Categories
Please, save `memberInFocus.DataContext as BrowserInternalElement` in local variable. Is it instead of a number of casts.