Namespace can have classes and members at the same time

#### Purpose

Before PR logic doesn't support situation when namespace can have members directly. In UI you can see this.
![dualitybefore](https://cloud.githubusercontent.com/assets/8158551/4889757/e9867db2-6398-11e4-8536-75fbb4b26cb8.PNG)

PR fixes the logic. now you will see it.
![dualityafter](https://cloud.githubusercontent.com/assets/8158551/4889739/b5cf5890-6398-11e4-84d7-c8cd9795b1d8.PNG)

#### Modifications

To `CategoryBuilder.AddCategory` is added handler of situations when we have paths like `Analyze.Render` namespace and `Analyze.Classes.Render` class. Children of second class are moved to `Analyze.Render` namespace.

#### Additional references

[MAGN-5259](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5259).

Styling will be done in next PR.

#### Reviewers

@aosyatnik, @Benglin, please, take a look.
`Items` is `ObservableCollection<>`. It doesn't have `AddRange()` function.