Add QGuiApp::topLevelAt and fix visibility flag

Add and remove windows to the list of top level windows.
Correctly set the visibility flag for windows and implement
a QGuiApplication::topLevelAt() method that uses the
QPlatformScreen.

src/gui/kernel/qguiapplication.cpp

@@ -183,6 +183,27 @@ QWindowList QGuiApplication::topLevelWindows()
     return QGuiApplicationPrivate::window_list;
 }
 
+QWindow *QGuiApplication::topLevelAt(const QPoint &pos)
+{
+    QPlatformIntegration *pi = QGuiApplicationPrivate::platformIntegration();
+
+    QList<QPlatformScreen *> screens = pi->screens();
+    QList<QPlatformScreen *>::const_iterator screen = screens.constBegin();
+    QList<QPlatformScreen *>::const_iterator end = screens.constEnd();
+
+    // The first screen in a virtual environment should know about all top levels
+    if (pi->isVirtualDesktop())
+        return (*screen)->topLevelAt(pos);
+
+    while (screen != end) {
+        if ((*screen)->geometry().contains(pos))
+            return (*screen)->topLevelAt(pos);
+        ++screen;
+    }
+    return 0;
+}
+
+
 static void init_platform(const QString &name, const QString &platformPluginPath)
 {
     QGuiApplicationPrivate::platform_integration = QPlatformIntegrationFactory::create(name, platformPluginPath);

src/gui/kernel/qguiapplication.h

@@ -80,6 +80,7 @@ public:
     virtual ~QGuiApplication();
 
     static QWindowList topLevelWindows();
+    static QWindow *topLevelAt(const QPoint &pos);
 
     static QWindow *activeWindow();
 

src/gui/kernel/qwindow.cpp

@@ -58,21 +58,27 @@ QWindow::QWindow(QWindow *parent)
 {
     Q_D(QWindow);
     d->parentWindow = parent;
+    QGuiApplicationPrivate::window_list.prepend(this);
 }
 
 QWindow::~QWindow()
 {
     if (QGuiApplicationPrivate::active_window == this)
         QGuiApplicationPrivate::active_window = 0;
+    QGuiApplicationPrivate::window_list.removeAll(this);
     destroy();
 }
 
 void QWindow::setVisible(bool visible)
 {
     Q_D(QWindow);
-    if (!d->platformWindow) {
+
+    if (d->visible == visible)
+        return;
+    d->visible = visible;
+
+    if (!d->platformWindow)
         create();
-    }
     d->platformWindow->setVisible(visible);
 }