Further move onCloseEvent to base class

So that it can be reused by MDI layouts too

src/private/DropArea.cpp

@@ -397,15 +397,3 @@ DockWidgetBase *DropArea::mdiDockWidgetWrapper() const
 
     return nullptr;
 }
-
-void DropArea::onCloseEvent(QCloseEvent *e)
-{
-    e->accept(); // Accepted by default (will close unless ignored)
-
-    const Frame::List frames = this->frames();
-    for (Frame *frame : frames) {
-        qApp->sendEvent(frame, e);
-        if (!e->isAccepted())
-            break; // Stop when the first frame prevents closing
-    }
-}

src/private/DropArea_p.h

@@ -77,8 +77,6 @@ public:
 
     /// Returns the helper dock widget for implementing DockWidget::Option_MDINestable.
     DockWidgetBase *mdiDockWidgetWrapper() const;
-
-    void onCloseEvent(QCloseEvent *);
 private:
     Q_DISABLE_COPY(DropArea)
     friend class Frame;

src/private/LayoutWidget.cpp

@@ -302,3 +302,15 @@ LayoutSaver::MultiSplitter LayoutWidget::serialize() const
 
     return l;
 }
+
+void LayoutWidget::onCloseEvent(QCloseEvent *e)
+{
+    e->accept(); // Accepted by default (will close unless ignored)
+
+    const Frame::List frames = this->frames();
+    for (Frame *frame : frames) {
+        qApp->sendEvent(frame, e);
+        if (!e->isAccepted())
+            break; // Stop when the first frame prevents closing
+    }
+}

src/private/LayoutWidget_p.h

@@ -189,6 +189,8 @@ public:
     virtual bool deserialize(const LayoutSaver::MultiSplitter &);
     LayoutSaver::MultiSplitter serialize() const;
 
+    void onCloseEvent(QCloseEvent *) override;
+
 protected:
     void setRootItem(Layouting::ItemContainer *root);
     /**