Allow each QDockWidget to store their state separately (2)

I have seen that QDockWidget was requested for a different reason before: https://bugreports.qt.nokia.com/browse/QTBUG-2824. As the cause was removed ( need for dynamically created docks to be saved properly ) the feature request was closed.

Allowing to save the state of a QDockWidget may be useful for other reasons, however. For example applications implementing workspaces that allow customisation of their appearance and content (think about plug-ins creating the dock panels) would need a way to save that states ( that could be achieved using QMainWindow::saveState() ) but also to restore that state based on the content of saved data.