DUI: TreeView and TemplateSelector for NestedClasses added.

# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4555869/07054d18-4ec3-11e4-96d7-856ccfa0ce3e.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4555876/23515cf0-4ec3-11e4-9d27-b596ccbcfede.png)
![image](https://cloud.githubusercontent.com/assets/8158404/4555898/6586cd94-4ec3-11e4-9599-71cc80eb9622.png)

# Solution
New TemplateSelector has been created.
# Reviewers
@Benglin,@vmoyseenko, please take a look.
Tasks on YouTrack:
[MAGN-4691](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4691) DUI: Try to replace Categories ListView to TreeView
[MAGN-4693](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4693) DUI: Create TemplateSelector which shows correct DataTemplate for TreeView element