DUI: key navigation. Part 5(or 2.1)

State: _High_
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
@Benglin ,@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.
If we rewrite the above code change in this way, it makes the intents much clearer:

```cs
if (e.Key == Key.Up)
{
    if (classButton.DataContext is NodeSearchElement)
        // Bla bla bla...
}
else if (e.Key == Key.Down)
{
    if (classButton.IsSelected)
        // Bla bla bla...
}
```

Does that make sense?