A single button widget for toggling vr mode.
    
| Name | Type | Default | Description | 
|---|---|---|---|
container | 
            
            Element | String | The DOM element or ID that will contain the widget. | |
scene | 
            
            Scene | The scene. | |
vrElement | 
            
            Element | String | 
                
                    document.body
                
                 | 
            
            optional The element or id to be placed into vr mode. | 
Throws:
- 
    DeveloperError : Element with id "container" does not exist in the document.
 
Members
    Gets the parent container.
viewModel : VRButtonViewModel
    Gets the view model.
Methods
    Destroys the widget.  Should be called if permanently
removing the widget from layout.
Returns:
    true if the object has been destroyed, false otherwise.
    
