[scene] Make the scene window a qobject

Since the scene window is not a QObject, we cannot connect toplevel's
signals directly to the scene window's slots.

plugins/scenes/opengl/scene_opengl.h

@@ -147,6 +147,8 @@ class OpenGLWindowPixmap;
 
 class OpenGLWindow final : public Scene::Window
 {
+    Q_OBJECT
+
 public:
     enum Leaf { ShadowLeaf, DecorationLeaf, ContentLeaf, PreviousContentLeaf };
 

plugins/scenes/qpainter/scene_qpainter.h

@@ -91,6 +91,8 @@ private:
 
 class SceneQPainter::Window : public Scene::Window
 {
+    Q_OBJECT
+
 public:
     Window(SceneQPainter *scene, Toplevel *c);
     ~Window() override;

plugins/scenes/xrender/scene_xrender.h

@@ -186,9 +186,10 @@ private:
     QScopedPointer<XRenderBackend> m_backend;
 };
 
-class SceneXrender::Window
-    : public Scene::Window
+class SceneXrender::Window : public Scene::Window
 {
+    Q_OBJECT
+
 public:
     Window(Toplevel* c, SceneXrender *scene);
     ~Window() override;

scene.cpp

@@ -703,8 +703,9 @@ QVector<QByteArray> Scene::openGLPlatformInterfaceExtensions() const
 // Scene::Window
 //****************************************
 
-Scene::Window::Window(Toplevel * c)
-    : toplevel(c)
+Scene::Window::Window(Toplevel *client, QObject *parent)
+    : QObject(parent)
+    , toplevel(client)
     , filter(ImageFilterFast)
     , m_shadow(nullptr)
     , m_currentPixmap()

scene.h

@@ -286,11 +286,13 @@ protected:
 };
 
 // The base class for windows representations in composite backends
-class Scene::Window
+class Scene::Window : public QObject
 {
+    Q_OBJECT
+
 public:
-    Window(Toplevel* c);
-    virtual ~Window();
+    explicit Window(Toplevel *client, QObject *parent = nullptr);
+    ~Window() override;
     // perform the actual painting of the window
     virtual void performPaint(int mask, const QRegion &region, const WindowPaintData &data) = 0;
     // do any cleanup needed when the window's composite pixmap is discarded