ScaleButton: Correct the constructor.

gtk_scale_button_new() does more than just call g_object_new() so
we cannot just assume that the parameters correspond to
GObject property names.
This has apparently been wrong from the start, many years ago.
Probably few people, if any, need to use this.
ScaleButton: Add a constructor without the icons.

Because gtk_scale_button_new() allows that to be NULL.
ScaleButton: Document the constructors.