Function windowClassOf

this is a helper function that takes the logic of determining window classes and puts it all in one place there are a number of places where we need to know what window class an arbitrary handle has theoretically we could use the class atom to avoid a _wcsicmp() however, raymond chen advises against this - http://blogs.msdn.com/b/oldnewthing/archive/2004/10/11/240744.aspx (and we're not in control