message: Ensure setting one active removes the other
  If the Avogadro GL widget is active the VTK widget cannot be, and
  vice-versa. This ensures that we can figure out what is active even if
  it is a little convoluted.
diff: @@ -37,6 +37,7 @@ QtGui::Molecule* ActiveObjects::activeMolecule() const
  void ActiveObjects::setActiveGLWidget(GLWidget* glWidget)
  {
  if (m_glWidget != glWidget) {
  + m_widget = nullptr;
  m_glWidget = glWidget;
  emit activeGLWidgetChanged(m_glWidget);
  setActiveWidget(glWidget);
  @@ -46,6 +47,7 @@ void ActiveObjects::setActiveGLWidget(GLWidget* glWidget)
  void ActiveObjects::setActiveWidget(QWidget* widget)
  {
  if (m_widget != widget) {
  + m_glWidget = nullptr;
  m_widget = widget;
  emit activeWidgetChanged(widget);
  }
  