DUI: Addons button.

Priority: _Normal_
Addons button was created based on this picture:
![image](https://cloud.githubusercontent.com/assets/8158404/5026620/9bf0ac60-6b2e-11e4-8d16-f83a27ce10e3.png)

Result:
![image](https://cloud.githubusercontent.com/assets/8158404/5026647/cdf305b4-6b2e-11e4-9293-d0fff026e1db.png)


Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4649](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4649) DUI: Add and style "Import add-on" button