Children should be collapsed during expanding of parent

#### Purpose

After opening of namespaces all subnamespaces and StandardPanel should be closed, no any class selected.

Example 
We got next view. `Core.File` namespace is expanded. `Core.Types` class expanded.
![image](https://cloud.githubusercontent.com/assets/8158551/5359441/158e022e-7fc6-11e4-9b65-9ea4d1a550fc.png)

if you reopen `Core` (click double on category) you should get this view
![image](https://cloud.githubusercontent.com/assets/8158551/5359482/6f489798-7fc6-11e4-9fa7-f12bb4908334.png)

#### Modifications

Added collapsing of next level items in `ToggleIsExpanded` command.

#### Additional references

[MAGN-5619](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5619).

#### Reviewers

@Benglin, please, take a look.
Can we change this to include curly brackets?

```csharp
if (endState)
{
    foreach (var ele in element.Items)
        ele.IsExpanded = false;
}
```