core/proxywindow: fix ProxiedWindow proxy pointer after reload

Previously was not updated after reload, causing QsWindowAttached to
use the old window pointer after it had been freed.

src/window/test/windowattached.hpp

@@ -9,6 +9,7 @@ class TestWindowAttachment: public QObject {
 private slots:
 	static void attachedAfterReload();
 	static void attachedBeforeReload();
+	static void earlyAttachReloaded();
 	static void owningWindowChanged();
 	static void nonItemParents();
 };