This is slightly simpler, as it doesn't require checking whether the
control state really changes or not (it always does if DoEnable() is
called) and allows disabling the controls before creating them, e.g.
code like
wxButton* const b = new wxButton();
b->Disable();
b->Create(this, wxID_OK);
works as expected now instead of spewing GTK+ errors.
96f3832d52