Update Visual Studio Build Files

Update Visual Studio Build Files

-Update the pre-configured config.h.win32(.in) to define _GDK_EXTERN as
 __declspec (dllexport) as we are not using .def files to export symbols
 anymore.
-Update the GDK/GTK DLL projects and the property sheets to stop using
 the .symbols/.def files
-Update the property sheets to "install" the newly-introduced GTK headers
-Update the gtk3-demo project to build the new demo sources that must be
 built