Fix MainWindow not propagating close events to docked widgets

Nested FloatingWindows already supported it, so make it consistent.

Personal take: In a non-docking world, users can override their
main window close event and prevent a close, to save a document
or such. However, in a docking world, the main window developer
won't know which widgets are docked, so forwarding needs to happen,
as some might have documents to save.

src/MainWindow.h

@@ -57,6 +57,7 @@ public:
     void setCenterWidgetMargins(const QMargins &margins);
 
 protected:
+    void closeEvent(QCloseEvent *) override;
     void resizeEvent(QResizeEvent *) override;
     QRect centralAreaGeometry() const override;