Improve safety in code (#588)

Delete areas later so that that they can be accessed by
(inherited) dock widgets in dtor. Add some QPointer to
prevent crashes.

Hence allow users to do more while dock widgets etc
are being destroyed.

src/DockWidget.cpp

@@ -81,7 +81,7 @@ struct DockWidgetPrivate
 	CDockWidgetTab* TabWidget = nullptr;
 	CDockWidget::DockWidgetFeatures Features = CDockWidget::DefaultDockWidgetFeatures;
 	CDockManager* DockManager = nullptr;
-	CDockAreaWidget* DockArea = nullptr;
+	QPointer<CDockAreaWidget> DockArea;
 	QAction* ToggleViewAction = nullptr;
 	bool Closed = false;
 	QScrollArea* ScrollArea = nullptr;