DUI: toggle button color change for nested classes

# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4573414/18214190-4f92-11e4-9aed-190b69166536.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4573398/f2ef4c00-4f91-11e4-8d9e-ab61d0954417.png)
# Solution
Depending on type we choose background of toggle button. If item is `RootElement` Background will be dark grey. If item is `InternalElement` Background will be light grey.
# Reviewers
@Benglin,@vmoyseenko, please take a look.

Tasks on YouTrack:
[MAGN-4966](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4966) DUI: Styling Toggle Button for Nested Category
Since you're working on this one, can we rename ```DataContextToBoolConverter``` to ```ElementTypeToBoolConverter```? That sounds more appropriate. Thanks!