Namespaces style modification

#### Purpose

Styling Addons TreeView as EB suggested. Refer to [document](https://docs.google.com/document/d/1LZgwif8VcmjyFR3Eq9s2In-EFbyp6EfGQWO6clkVoZc/edit#bookmark=id.qtg1wb6kixpk). PR implements options 1 and 2.

#### Modifications

XAML changes. Result:
![image](https://cloud.githubusercontent.com/assets/8158551/4975016/d5622a32-68db-11e4-90ca-f49b94ad6243.png)

#### Additional references

[MAGN-5342](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5342).
[MAGN-5343](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5343).

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
This ```Style``` is exactly the same as the one above, can we not have a named ```Style``` and reused it for these two places?