add first basic implementation of a collapsible dock widget. This can be used in two ways; either you can replace

QDockWidget with KoDockWidget to get a collapsible dock widget, or you can continue using QDockWidget and add a setTitleBarWidget(new KoDockWidgetTitleBar(...)) call to set a title bar widget that should be identical to the default tile bar except that it has one additional button on the far left that can be used to collapse the dockwidget.