DUI: indentation nested class toggle button

# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4558444/78b43f60-4ee0-11e4-858e-5a11e53b451a.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4558433/4e67a454-4ee0-11e4-914b-3c7f80355988.png)
# Solution
I used `FullCategoryName`. So that, number of points in `FullCategoryName` indicates what margin should we use.
# Reviewers
@Benglin,@vmoyseenko, please take a look.
Tasks on YouTrack:
[MAGN-4966](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4966) DUI: Styling Toggle Button for Nested Category
Please use ```string.IsNullOrEmpty(incomingString)``` to cover the empty string case also.