Move Custom Nodes to regular libraries tree

#### Purpose

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.
There are problems with this method, the last one stops me from merging this in:

1. The return type ```FrameworkElement``` does not match ```ListBoxItem```, as the function name suggests, it should be a ```ListBoxItem```.
2. The function name can be just ```GetSelectedListBoxItem```.
3. We could use multiple lines to shorten the code.
4. The method can and should be made ```static``` (which will make problem 5 obvious).
5. ```topResultListBox``` is being used in the method instead of ```listbox```.

Here's what I'd suggest:

```csharp
private ListBoxItem GetSelectedListBoxItem(ListBox listbox)
{
    if (!listbox.HasItems || (listbox.SelectedIndex == -1))
        return null;

    var generator = listbox.ItemContainerGenerator;
    return generator.ContainerFromItem(listbox.SelectedItem) as ListBoxItem;
}
```