Detach when double-click on tab bar background, if no title bar

When a title bar isn't visible, we need a way to detach an entire
group of tabs by double clicking, required for wayland.

Implemented for all platforms, as it makes sense

src/private/widgets/TabWidgetWidget_p.h

@@ -41,6 +41,7 @@ public:
     int indexOfDockWidget(DockWidgetBase *) const override;
 protected:
     void paintEvent(QPaintEvent *) override;
+    void mouseDoubleClickEvent(QMouseEvent *) override;
     void tabInserted(int index) override;
     void tabRemoved(int index) override;
     bool isPositionDraggable(QPoint p) const override;