Built-in functions/Operators should be a linear list 

#### Purpose

"Built In functions" and "Operations" root categories don't have subcategories just action members. So we should for them or any other similar root categories show `StandardPanel` instead of subcategories ListView as it now done.

#### Modifications

Added property `IsPlaceHolder` to `BrowserRootElement` to specify whether it has children or not. Used for population of `ClassDetails` property of `BrowserRootElement`.

To `LibraryView.xaml` added new `ContentControl`. By new `SubclassesTemplateSelecter` it is decided what to show: `ListView` with subcategories or `StandardPanel` with memberlists. Both views presented as `DataTemplates`.

#### Additional references

In Youtrack tracked as [MAGN-4352](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4352).

#### Reviewers

@Benglin, please take a look.


Could we group this ```DataTemplate``` alongside ```ClassObjectDataTemplate``` and ```ClassDetailsDataTemplate```? Then I'd like you to document in the following way above each of these template to say what they do:

```xml
<!--
This data template is used to show bla bla bla.... An example of this 
data template use case is Operator and Built-In Function, bla bla bla...
-->
```