Move Custom Nodes to regular libraries tree

#### Modifications

Nodes under "Addons" header are moved to regular libraries tree. For both views: library view and search results.

##### Before
![001](https://cloud.githubusercontent.com/assets/8158551/5106932/e97b355a-7005-11e4-8c17-6417fc03eb0d.PNG)

##### After
![002](https://cloud.githubusercontent.com/assets/8158551/5106940/f4779624-7005-11e4-907a-34acabb626c4.PNG)

#### Modifications

All nodes are added using `BrowserCategoriesBuilder`. Removed some code which are special for addons representation.

#### Additional references

[MAGN-5483](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5483).

There will be removed more redundant code as EB said he liked it. The task for it: [MAGN-5484](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5484).

#### Reviewers

@Benglin, please, take a look.
Sorry @vmoyseenko, I think you have missed another point in the original comment. What I mean was, instead of using ```WPF.FindChild```, can we not do this:

```csharp
// Note that I made up the following methods because 
// I'm typing in Notepad++, but you get the idea.
HighlightedItem = null;
if (topResultListBox.HasItem())
    HighlightedItem = topResultListBox.FirstItem();
```

Also, it's possible for the ```topResultListBox``` to contain no items at all, that might crash Dynamo. We need to guard against ```HighlightedItem``` being ```null``` below.