Called by the framework when the user activates a worksheet
[C++]
virtual void OnSheetActivate(
   LPDISPATCH dispWorksheet
);
Pointer to dispatch interface of type Excel.Worksheet.
This function is called whenever the user activates a worksheet. Note that it will only be called if event-sinking is switched on 
            (using CXllApp::SetSinkEvents()). You can override the method in your application class
            to react to the event.
            The default method does nothing.
You should only implement this function if you need access to Excel's COM interfaces.
			If you just want to call an add-in function, use the much simpler mechanism 
			provided by CXllApp::SetComEventHandler().
			
Header: xllplus.h
CXllApp Class 
					  | CXllApp Methods			 
					  | CXllApp::OnSheetDeactivate()			 
					  | CXllApp::OnWorkbookActivate()			 
					  | CXllApp::OnSheetSelectionChange()