ui: Properly handle sub window stacks with QtWindowStackElement interface

src/lib_gui/qt/window/QtWindowStack.h

@@ -5,6 +5,17 @@
 
 #include <QWidget>
 
+class QtWindowStackElement
+	: public QWidget
+{
+public:
+	QtWindowStackElement(QWidget* parent = nullptr);
+
+	virtual void showWindow() = 0;
+	virtual void hideWindow() = 0;
+};
+
+
 class QtWindowStack
 	: public QObject
 {
@@ -18,15 +29,15 @@ signals:
 public:
 	QtWindowStack(QObject* parent = nullptr);
 
-	QWidget* getTopWindow();
+	QtWindowStackElement* getTopWindow();
 
 public slots:
-	void pushWindow(QWidget* window);
+	void pushWindow(QtWindowStackElement* window);
 	void popWindow();
 	void clearWindows();
 
 private:
-	std::vector<QWidget*> m_stack;
+	std::vector<QtWindowStackElement*> m_stack;
 };
 
 #endif // QT_WINDOW_STACK