DUI: Library button back to visible fix

# Preface
Steps to reproduce:
1. Run Dynamo.
2. Open any category full with class buttons.
e.g. Core.
3. Collapse library column with this button.
![image](https://cloud.githubusercontent.com/assets/8158404/4354596/df259efe-423f-11e4-93e5-d675a9dc458f.png)
4. Open library collum with this button.
![image](https://cloud.githubusercontent.com/assets/8158404/4354606/ec9dbb34-423f-11e4-9c8e-56179cfed5b1.png)
5. Dynamo got crashed.

# Reviewers
@Benglin,@vmoyseenko, please take a look.
Yah I do think the current code for ```virtual Items``` is a little weird, I'll accept this for now (I think the codes in ```master``` needs a little tidying up).