Windows: Update code for monolithic GTK DLL

Windows: Update code for monolithic GTK DLL

Now that the autotools build folded the GDK/GSK bits into the main GTK+
DLL, there are some updates that need to be done for this.  We need to:

-Fold the DllMain() of GDK-Win32 into the main GTK+ DllMain(), as we need
 the HINSTANCE to register the window.  We can't have two DllMain()'s in a
 single DLL.
-Remove the GDK rc(.in) files, as that is not used anymore.  Make the GTK+
 .rc(.in) file load the gtk.ico GTK+ logo file instead so that we still
 get the GTK+ logo for the application icon by default.  Update the
 autotools build files as well.
-Revert commit b9f9980 as LRN pointed out in comment 25 in bug 773299, as
 GTK+ is now a monolithic DLL, and we ought not to export this private
 function.

https://bugzilla.gnome.org/show_bug.cgi?id=773299