external add_accelerator :
([>`widget] as 'a) obj -> sgn:('a,unit->unit) GtkSignal.t ->
accel_group -> key:Gdk.keysym -> ?modi:Gdk.Tags.modifier list ->
?flags:accel_flag list -> unit
= "ml_gtk_widget_add_accelerator_bc" "ml_gtk_widget_add_accelerator"