Constructs a CXlSelectionChangeEventStaticObserver object
During construction, the observer is added to a linked list of static observers. Note that this linked list is safe to use at any time, so this class can be used for static variables.
At the appropriate time, the framework will register the observer with its event, and later will unregister it. For most events, registration occurs during XllOpen; for the XllOpen and XlRegisterFunctions events, registration occurs during InitInstance.
See the Update method for an example of this class in use.