DUI: reveal all items under label

State: _High_
# Preface
All members should appear after click on Group. Group should be highlighted, when mouse is over, and underlined after selection.
# Before
![image](https://cloud.githubusercontent.com/assets/8158404/4766417/3292b09e-5b50-11e4-9ffb-122c9360622d.png)
# After
![image](https://cloud.githubusercontent.com/assets/8158404/4766420/3df10d3c-5b50-11e4-8c98-40e41cee4690.png)
# Reviewers
@Benglin ,@vmoyseenko, please take a look.

Link to YouTrack:
[MAGN-4919](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4919) DUI: On clicking each of the labels of results, they should reveal all items under that label
It's generally a better *API design practice* to not return a ```null``` for any function that returns a collection, so we should return ```new List<BrowserInternalElement>``` here.

As for the next part... I'm imagining ```Parent.Items``` to be returning those sibling items of ```firstMember```, do we really need to check for the same parent group among the sibling items? Do you think the following will work? Also, is it possible to remove the ```ToList``` call?

```cs
var siblings = firstMember.Parent.Items.OfType<BrowserInternalElement>();
return siblings;
```

If that won't work, then maybe we need to add some comments here so it is immediately clear why we need such query. Thanks!