Add LayoutSaver::restoredDockWidgets()

Implemented as a property it's better that the LayoutSaver
doesn't carry state about this. Then we would need to remove dock
widgets from the list if they are deleted. Also not a important
enough feature to be on the DockWidget public interface.

src/DockWidgetBase.cpp

@@ -557,6 +557,7 @@ DockWidgetBase *DockWidgetBase::createFromDataStream(QDataStream &ds)
     if (dw) {
         if (QWidget *w = dw->widget())
             w->setVisible(true);
+        dw->setProperty("kddockwidget_was_restored", true);
     } else {
         qWarning() << Q_FUNC_INFO << "Couldn't find dock widget" << name;
     }