BEAST-GTK: work around regressions introduced in  g_object_class_install_property
BEAST-GTK: work around regressions introduced in  g_object_class_install_property

See Gnome bug #686652, g_object_class_install_property falsely forces
properties to be readable or writable.

swesterfeld/beast,GNOME/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,GNOME/beast,swesterfeld/beast,GNOME/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast