Dui key navigation(class button enter key down)

Yellow navigation is from [first PR](https://github.com/Benglin/Dynamo/pull/123).
Light pink is from [second PR](https://github.com/Benglin/Dynamo/pull/127).
Red is from this PR.
This PR makes navigation between methods inside class possible.
![image](https://cloud.githubusercontent.com/assets/8158404/4735063/e9b1c4f2-59db-11e4-9e88-bc013cfd89be.png)
Reviewers
@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5075](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5075) DUI: On Enter for Class the list of members should be collapsed / expanded.
Rewrite, please, as `classButton.IsSelected=!classButton.IsSelected`.