thread) which are called whenever the throbber should be
animated/busy or idle. *)
+val set_statusbar : window_state -> string -> unit
+ (** This callback from the slave thread (invoked in the main thread)
+ updates the status bar when some slave operation starts or
+ stops. *)
+
val progress : window_state -> int64 * int64 -> unit
(** This called whenever the progress bar should move. *)