DUI: highlighting toggle triangle

We found out, that triangle isn't highlighted only for root categories, that are full of classes. For others(like Operators) everything worked properly. So, I've decided to update xaml styles.

Reviewers
@Benglin,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-5087](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5087) DUI: On hovering category style is not changed for core libraries
We have 2 types of root categories:
1. Like Nested classes (e.g. Core, Geometry).
2. Like Expanders (e.g. Operators, BuiltIn).
They use different templates, but look identically.

Let's name 1st type - nested category and 2nd - expander category.