Windows: Update how gtk-win32.rc is generated

On Visual Studio, unlike MinGW, manifest files are embedded via
including the manifest file as a resource file in the projects, not
via the .rc file.  This means that the line in the .rc file that
specifies the manifest file would cause trouble, so that line gets
removed when the full gtk3-win32.rc is generated on Visual Studio builds,
otherwise 2010+ Visual Studio will complain when compiling the .rc file.
Also, the inclusion of winuser.h will cause warnings during the
compilation of the .rc file.

Fix this by isolating the Win32 resource portions of gtk-win32.rc.in to
gtk-win32.rc.body.in and:
-On MinGW, construct the full gtk-win32.rc by doing the winver.h and
 winuser.h inclusion first, then append the contents of gtk-win32.rc.body,
 and then appending the line to embed the manifest file.
-On Visual Studio, simply copy the gtk-win32.rc.body to gtk-win32.rc,
 and generate the full libgtk3.manifest file.

https://bugzilla.gnome.org/show_bug.cgi?id=762311

configure.ac

@@ -1943,7 +1943,7 @@ gdk/gdkversionmacros.h
 gtk/Makefile
 gtk/makefile.msc
 gtk/gtkversion.h
-gtk/gtk-win32.rc
+gtk/gtk-win32.rc.body
 gtk/libgtk3.manifest
 libgail-util/Makefile
 modules/Makefile