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.
Minor thing (you don't have to change), but I'd have done it this way :)

```cs
var baseUrl = @"pack://application:,,,/DynamoCore;component/UI/Images";

private BitmapImage searchIconBitmapNormal = 
    new BitmapImage(new Uri(baseUrl + "search_normal.png"));
private BitmapImage searchIconBitmapHover = 
    new BitmapImage(new Uri(baseUrl + "search_hover.png"));
```