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.

tests/tst_docks.cpp

@@ -2989,8 +2989,11 @@ void TestDocks::tst_preventClose()
     fw->close();
     QVERIFY(dock1->isVisible());
 
-    dock1->deleteLater();
-    QVERIFY(Testing::waitForDeleted(dock1));
+    // Put into a main window
+    auto m = createMainWindow();
+    m->addDockWidget(dock1, KDDockWidgets::Location_OnRight);
+    m->close();
+    QVERIFY(dock1->isVisible());
 }
 
 void TestDocks::tst_propagateMinSize()