DUI: Classes must be always at top of panel

So, I found out, that before loading SearchModel it makes sorting of children by name. Luckily, order of BIE and BIEFC was right in standalone application , but not in Revit.
That's why I added case, so that now at the top level application sets only BIEFC, if BIEFC is presented.

Reviewers:
@Benglin, please take a look.
Link to YouTrack:
[MAGN-5016](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5016) DUI: Classes must be always at top of panel with classes and nested categories
Hi @aosyatnik, please rephrase this to:

```cs
// BrowserInternalElementForClasses object, if any, must 
// always appear before any other nested namespaces. 
```

If I don't get to merge this tonight I'll do so tomorrow morning :)