Use property system to coerce model data to G_TYPE_STRING
Use property system to coerce model data to G_TYPE_STRING

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=423201

johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,Unity-Technologies/gtk