DUI: view model change(for nested classes)

# Preface
I have added new class `BrowserInternalElementForClasses`. It's used to store classes separately from nested classes.
Also I've added some tests.

# Reviewers
@Benglin,@vmoyseenko, please take a look.

Links to YouTrack:
[MAGN-4869](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4869) DUI: Build tree of nodes using BrowserInternalElementForClasses class
[MAGN-4868](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4868) DUI: Create and/or update unit tests to test new tree of nodes
[MAGN-4867](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4867) DUI: Create BrowserInternalElementForClasses which is the container of classes

Hmm... I can't get rid of this setter. In `BrowserItem` it's written 
```public abstract ObservableCollection<BrowserItem> Items { get; set; }```. The same is with `IEnumerable`.