code: def _lock_aspect_cb(self, w, tf):
          
          self._lock_aspect = tf
          self.w.aspect.set_enabled(tf)
          if self._lock_aspect:
              self._set_aspect_cb()
          else:
              wd, ht = self.get_wdht()
              _as = self.calc_aspect_str(wd, ht)
              self.w.aspect.set_text(_as)
docs: This function is called when the user clicks the 'Lock aspect'
          checkbox.  `tf` is True if checked, False otherwise.