Background of StandardPanel for no classes structure should be different

#### Purpose

According to [Library visual spec](https://docs.google.com/document/d/1ca3XX9QKzzt1cExAwCD0mnyb5pZWpFcYAoz88jJNXyk/edit#bookmark=id.xkil8fhwg4wi) `StandardPanel` has different background for no classes structure. Examples of no classes structure are "Operators" and "Builtin Functions".

#### Modifications

Was created property `ClassInforamtion.IsRootCategoryDetails`. Using this property correct background is set for:

* `Border` where body of `StandardPanel` is situated;
* `ListBoxItem` where every member is situated.

#### Additional references

[MAGN-4764](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4764).

#### Reviewers

@aosyatnik, @Benglin please take a look.
Yes. `ClassInformation` is DataContext of `StandardPanel`. `IsPlaceHolder`situated is property of `BrowserRootElement`. We do not have access to `IsPlaceHolder` from DataContext of `StandardPanel`.