[duim,gtk] Create radio-menu-buttons as GtkRadioMenuItems

When creating the radio buttons we assign the correct group.
Also adds the necessary binding in gtk-dylan.
[duim,gtk] Correctly handle menu buttons activate signal

Just calling activate-gadget on a gtk-radio-menu-button is not enough to
set the menu-box's selection to the activated item. Calls to gadget-value
were always returning #f because the gadget's box current selection was not
set.
[duim,reversi] Use correct keyword filled?: