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!
Bad idea to check by type. It is possible we will have to ListBoxes in future. Can you please find other way to know what it is: class or member?