DUI: Empty place appears for StandardPanel if top group is only presented

# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4543688/6efb4922-4e2d-11e4-930a-a94451c9e283.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4543697/81326e68-4e2d-11e4-9809-5bc74efb5006.png)
# Reviewers
@Benglin, please take a look.

Link to YouTrack:
[MAGN-4950](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4950#) DUI: Empty place appears for StandardPanel if top group is only presented