MAGN-4584: Centering class buttons

# Preface
On resizing, the icon squares flow. Arrange the icons such that they are center aligned horizontally to the big container, with same space on both side. For example
![image](https://cloud.githubusercontent.com/assets/8158404/4271663/e340ccc6-3cda-11e4-9a7f-8951b23b39c2.png)
 All gaps are of same length (tolerance 1px)
# Solution
Arrange children in LibraryWrapPanel.
# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4271732/97063318-3cdb-11e4-9974-4b042bcbdb52.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4271699/492cf17c-3cdb-11e4-940c-fed7ca543d83.png)
#Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4584 DUI: During resize in StandardPanel subcategories should be centered at their imaginative columns](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4584)