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.
```cs
// This index is used to keep track of the selection index before selection 
// changes. It is also used to guard against unnecessary "OrderListItems" 
// calls which repaints the UI. Here its value is set to "-2" to ensure that 
// "OrderListItems" method gets called at least once at the beginning (i.e. 
// when "ListView.SelectedIndex" is "-1", which would have avoided the first
// "OrderListItems" if "currentSelectedIndex" is set to "-1").
// 
private int currentSelectedIndex = -2;
```