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.


As outlined below we don't really need this additional data member ```isPlaceholder```. Also, it should be ```Placeholder``` one word, not ```PlaceHolder``` as two words.