filechooserbutton: In SELECT_FOLDER, when the selection is empty, show (None) in the combo box
filechooserbutton: In SELECT_FOLDER, when the selection is empty, show (None) in the combo box

We do this by adding a new row type, ROW_TYPE_EMPTY_SELECTION.

Signed-off-by: Federico Mena Quintero <4999915a961edfd7686112c2935288e1266eae14@gnome.org>

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