DUI: open one class for one category

# Preface
Avoid situations presented on screenshot. Can be opened just one Class in one Category.
![image](https://cloud.githubusercontent.com/assets/8158404/4900112/f9dcf9fe-6422-11e4-90e4-7d6d275c8c9c.png)

# Solution
I think we need `IsSelected` property, that would be bound in two way mode. That's what I've done...

# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5272](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5272) DUI: For one category on class can be opened at the point
Do we need this cast?