Key navigation between member groups and list of classes.

Yellow navigation from [first PR](https://github.com/Benglin/Dynamo/pull/123).
Pink is navigation from this PR.
![image](https://cloud.githubusercontent.com/assets/8158404/4647175/6ae26922-5479-11e4-8ff8-5a8e62634b5a.png)
I left a lot of comments, but I don't know whether they make my code more readable.

@vmoyseenko , I would be really grateful, if you could review my second part of key navigation. I'm trying to do my best, but it's not so easy.
Thank you in advance!
I think this `if` should be moved after checking `e.Key == Key.Down`. We so not need it before this check.