app: some GimpProgress cleanup

- change start() and set_text() to use "format" and "..." instead of
  "message", allowing to format progress messages in place
- s/cancelable/cancellable/
- move "cancellable" to be the second argument of start()

app/widgets/gimpprogressbox.h

@@ -39,7 +39,7 @@ struct _GimpProgressBox
   GtkBox     parent_instance;
 
   gboolean   active;
-  gboolean   cancelable;
+  gboolean   cancellable;
   gdouble    value;
 
   GtkWidget *label;