Fixed Undock Button getting hidden after undocking and docking again a floating widget that contains only one single visble dock area (#816)

src/DockContainerWidget.cpp

@@ -907,6 +907,7 @@ void DockContainerWidgetPrivate::addDockAreasToList(const QList<CDockAreaWidget*
 	{
 		DockArea->titleBarButton(TitleBarButtonClose)->setVisible(true);
 		DockArea->titleBarButton(TitleBarButtonAutoHide)->setVisible(true);
+		DockArea->titleBarButton(TitleBarButtonUndock)->setVisible(true);
 	}
 
 	// We need to ensure, that the dock area title bar is visible. The title bar