New TreeView is introduced for CustomNodes

#### Purpose

According to design [document](https://docs.google.com/document/d/1sPlXdh2vsxU7gVbW-iKm1s1JtGYNh9hnvbD8L7wiAww) CustomNodes should be filtered to the old new TreeView.
![addons](https://cloud.githubusercontent.com/assets/8158551/4372632/bf28d210-4325-11e4-8d69-544215f246f5.PNG).

In this PR on start of program all CustomNodes appears in TreeView. For this TreeView original styles are used. TreeView situated in rectangle.
![primpladdons](https://cloud.githubusercontent.com/assets/8158551/4372708/773606ca-4326-11e4-8d52-930e6e1dd0dc.PNG)

#### Modifications

To `SearchModel` is added new property `AddonRootCategories`. Here are saved root categories for CustomNodes tree. Also modified some functions to add CustomNodes to `AddonRootCategories`.

#### Additional references

[MAGN-4646](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4646).
[MAGN-4647](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4647).

TreeView is situated in `LibraryContainerView` for now. In [MAGN-4731](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4731) it is planned to move it to `LibraryView`.

#### Reviewers

@aosyatnik, @Benglin, please take a look.
Strictly speaking this is not a ```node```, so ```NodeType``` is not quite a good name for for this, ```ElementType``` would have been better since we're dealing with ```BrowserRootElement``` ultimately.