DUI: styling update

Styles updated according [LibraryVisualSpec](https://drive.google.com/folderview?id=0B15a1zl00aS-MGVqWHdlS2Rmdnc&usp=sharing_eil).
What has not done yet:
- Class buttons alignment, task on YouTrack [MAGN-4584](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4584);
- Transparent Scrollbar, task on YouTrack [MAGN-4583](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4583) .

Task on YouTrack:
[MAGN-4242 DUI: make styles of subcategory looking as in UI Proposal document](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4242)
Reviewers
@Benglin, please take a look.
Few things needed:

1. Please cache these as class data members: ```BitmapImage searchIconBitmap``` and ```SolidColorBrush searchForegroundBrush```.
2. Do not use ```BrushConverter```, simply do ```new SolidColorBrush``` with the color value.
3. Remember to freeze the brushes right after they are created: ```searchForegroundBrush.Freeze()```.