LayerManager.subscribeLayerOpacityChangeStart Method
        
 Subscribe an event to handle the Layer Opacity Change start event
 
         
        Parameters
        
            - unsubscribe
            
 
            - 
                
Type: Boolean
True to unsubscribe, False to subscribe. 
            - callback
            
 
            - 
                
Type: function (dundas.controls.LayerViewModel, float)
The event handler 
        
        Return Value
        
Type: LayerManager
        
This object.