Add Windows .bat and Python script to call g-ir-scanner to build
introspection files for Visual Studio builds.  This will read from the
autotools files using Python REGEX functionality to determine the headers
and sources for g-ir-scanner to process, so the autotools files will not
need to be updated except to distribute the necessary files.  Thils will
also enable one to build introspection files on Windows without using a
BASH-style shell such as MSYS.

Also add an utility Visual Studio project to call the Windows .bat to
build the introspection files for GTK+/GDK, for convenience.