gtk3: post-activate should call glib-compile-schemas

[gui-dos (Guido Soranzio)]

If you run "Pickers" from gtk3-demo the following error is reported when you try to choose a file:

GLib-GIO-ERROR **: No GSettings schemas are installed on the system

That's because the /opt/local/share/glib-2.0/schemas/org.gtk.Settings.FileChooser.gschema.xml has not been compiled: calling glib-compile-schemas is needed in the post-activate stage:

post-activate {
    system "${prefix}/bin/glib-compile-schemas \
        >${prefix}/share/glib-2.0/schemas"
}