DUI: addons styling

This PR is full of xaml changes, so I will leave comments in github.
I created styles based on this picture:
![image](https://cloud.githubusercontent.com/assets/8158404/4826423/3f5b61bc-5f70-11e4-8058-8e896f316f3a.png)

# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4826461/aa64be2c-5f70-11e4-8841-78d7dcebfd1b.png)

# After
![image](https://cloud.githubusercontent.com/assets/8158404/4826436/6a785f12-5f70-11e4-9e51-0d3acecb82d8.png)


# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5201](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5201) DUI: Styling of Addons TreeView items should be the same as categories of regular view
[MAGN-5204](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5204) DUI: Add 1px gap for Regular View, Addons categories
In this case I think the `StackPanel` above could be removed since it only has a single child element?