Adapt the Visual Studio project files to output the introspection files
in the same directories where the built binaries are located from the
previous patch.
Also, make the gtk3-introspect project dependent on the gdk-3 and gtk-3
projects only, so that we can build the introspection files without
needing to finish the whole build process. In order to "install" the
built introspection files, the gtk3-install project is now where this is
being done. Note that the introspection builds is still not built by
default at this point.
78fef66866