core/proxywindow: improve QsWindowAttached robustness

Can now track window parent window changes.
Added tests.

src/window/test/windowattached.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <qobject.h>
+#include <qtmetamacros.h>
+
+class TestWindowAttachment: public QObject {
+	Q_OBJECT;
+
+private slots:
+	static void attachedAfterReload();
+	static void attachedBeforeReload();
+	static void owningWindowChanged();
+	static void nonItemParents();
+};