DUI: popup menu for addons

Context menu has been made based on this picture:
![image](https://cloud.githubusercontent.com/assets/8158404/4972924/7a0bf06e-68b3-11e4-9ebc-26637bebc785.png)

Also region of DataTemplates moved under styles' region.

Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4656](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4656) DUI: Style popup menu
[MAGN-4655](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4655) DUI: Create popup menu