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.
I would like this comment to be updated this way:

// During search, backing data collections typically get updated frequently.
// This may result in corresponding BrowserInternalElement being removed or 
// updated. When that happens, the visual element 'HighlightedItem' that gets 
// bound to the original BrowserInternalElement then becomes DisconnectedItem.
// In such cases, we will reset the HighlightedItem to 'topResultListBox'.