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/MainWindowBase.cpp

@@ -39,6 +39,8 @@
 # include "DockWidget.h"
 #endif
 
+#include <QCloseEvent>
+
 using namespace KDDockWidgets;
 
 static LayoutWidget *createLayoutWidget(MainWindowBase *mainWindow, MainWindowOptions options)
@@ -791,3 +793,8 @@ QWidgetOrQuick *MainWindowBase::persistentCentralWidget() const
 
     return nullptr;
 }
+
+void MainWindowBase::onCloseEvent(QCloseEvent *e)
+{
+    d->m_layoutWidget->onCloseEvent(e);
+}