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
Let's not do this, few points:

1. We do not need additional nested ```Grid```, this is the reason we had outer ```Grid```.
2. We do not need ```Border``` and ```TextBlock```.
3. We need just a ```Button``` element using the same style as ```Run``` button.
4. Additional elements consume more memory and are slower to process.

What can be done:

1. Fix the ```Height``` of the second ```RowDefinition``` for the outer ```Grid```.
2. Make the new ```Button``` to be of fixed ```Width``` and ```Height```.
3. Make the new ```Button``` horizontally center-aligned, and vertically center-aligned.

Thanks!