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

build/win32/vs10/gtk3-demo.vcxproj

@@ -206,10 +206,13 @@
     <ClCompile Include="..\..\..\demos\gtk-demo\pickers.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\pixbufs.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\printing.c" />
+    <ClCompile Include="..\..\..\demos\gtk-demo\revealer.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\rotated_text.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\search_entry.c" />
+    <ClCompile Include="..\..\..\demos\gtk-demo\search_entry2.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\sizegroup.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\spinner.c" />
+    <ClCompile Include="..\..\..\demos\gtk-demo\stack.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\stock_browser.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\textscroll.c" />
     <ClCompile Include="..\..\..\demos\gtk-demo\textview.c" />