Visual C++ Builds: Build Introspection for GdkWin32

Visual C++ Builds: Build Introspection for GdkWin32

Add support to build the introspection files for GdkWin32, as done recently
in the autotools builds and clean up the NMake Makefile for building the
introspection files a bit.

For some reason, gdk_win32_display_manager_get_type() was not exported in
gdk-3.0.lib, force its export, so that the GdkWin32-3.0.gir can be built
properly with the Visual C++ builds.  This is a known problem that some
symbols in static libraries that are linked into a DLL in Visual C++, even
if they were marked with __declspec(dllexport) via _GDK_EXTERN.