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.


That's a clever use of ```IValueConverter```, good! I have some thoughts and I hope we can do it this way:

1. ```ClassDetails``` property is now constructed at the beginning, instead of on-demand. We should only construct it once when it is queried. So perhaps we can have ```classDetails``` data member as ```null``` one time, then when it is queried, create it by populating its member collections, cache its value, then return to the caller.

2. When the above is done, then we may not need ```ProcessRootCategories``` method at all, since we build this thing on-demand.