build: Integrate introspection builds into MSVC projects

build: Integrate introspection builds into MSVC projects

This is so that it is easier for one building GTK+ with Visual Studio to
build the introspection files as well in one shot.  Note that this is
not built by default, so one needs to select the gtk3-introspect
explicitly to build (and clean up) the introspection files.

For this to work, one needs to ensure the following:
-A complete build of GObject-Introspection in $(GlibEtcInstallRoot),
 that is built with the same installation of the Python interpretor that
 is used here (see PythonDir and PythonDirX64 in
 gtk3-version-paths.[vsprops|props]).
-Introspection files for ATK, GDK-Pixbuf and Pango, also in their proper
 locations under $(GlibEtcInstallRoot), which should be built with the
 same G-I installation.