screens into CPrimaryScreen and merged common code from secondary
screens into CSecondaryScreen. changed is-a relationship to a
has-a between the primary and secondary screen classes and the
generic platform dependent screen class to avoid multiple
inheritance of implementation. also standardized the interface
for those generic screen classes. adding a platform now involves
implementing simpler interfaces: IScreen for the generic screen,
IScreenEventHandler and some methods of CPrimaryScreen for the
primary screen, and IScreenEventHandler and some methods of
CSecondaryScreen for the secondary screen. did X11 platform
but not win32 platform.
