Dragging a dockwidget from a floating group to main window is buggy

If you have a floating group window with dock widgets and drag a dock widget and drop to one that's already docked in main window it becomes hidden

• run examples/widgets/mainwindows/mainwindow
• enable grouped dragging and disable animated
• Make two widgets float
• drag one of those over the other one, so you have a floating group with tabs
• drag a tab and drop it over an existing dock widget that is docked in main window

You'll notice the area gets tabs but it doesn't show anything.
This is because the dock widget we dropped is still parented to the floating group, because QDockAreaLayoutInfo::reparentWidgets() wasn't called.