This updates the Visual Studio Project GUIDs so that they don't repeat with
the GTK+-2.24.x ones, as the 3.x projects can be used with the 2.24.x in a
all-in-one solution file (such as when one wants to use a complete GTK+2
and GTK+3 stack when porting Windows applications from GTK+2 to GTK+3), and
each project in a solution file is expected to have an unique GUID.