This new design is simpler. For keyboard support, clients need only
implement 4 virtual methods on a class derived from CKeyState and
one trivial method in the class derived from CPlatformScreen, which
is now the superclass of platform screens instead of IPlatformScreen.
Keyboard methods have been removed from IPlatformScreen, IPrimaryScreen
and ISecondaryScreen. Also, all keyboard state tracking is now in
exactly one place (the CKeyState subclass) rather than in CScreen,
the platform screen, and the key mapper. Still need to convert Win32.
