Add focusWindowChanged signal to QGuiApplication

Required for Cocoa platform menus support, we need a
way to update state after focusWindow() result changes.

Change-Id: Idc573888c3d75bcbff2252e243c4b57b15fc2fcd
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>

src/gui/kernel/qguiapplication.cpp

@@ -520,6 +520,13 @@ QWindow *QGuiApplication::focusWindow()
     \sa focusObject()
 */
 
+/*!
+    \fn QGuiApplication::focusWindowChanged(QWindow *focusWindow)
+
+    This signal is emitted when the focused window changes.
+    \sa focusWindow()
+*/
+
 /*!
     Returns the QObject in currently active window that will be final receiver of events
     tied to focus, such as key events.
@@ -1337,6 +1344,8 @@ void QGuiApplicationPrivate::processActivatedEvent(QWindowSystemInterfacePrivate
         if (previousFocusObject != qApp->focusObject())
             self->q_updateFocusObject(qApp->focusObject());
     }
+
+    emit qApp->focusWindowChanged(newFocus);
 }
 
 void QGuiApplicationPrivate::processWindowStateChangedEvent(QWindowSystemInterfacePrivate::WindowStateChangedEvent *wse)

src/gui/kernel/qguiapplication.h

@@ -148,6 +148,7 @@ Q_SIGNALS:
     void screenAdded(QScreen *screen);
     void lastWindowClosed();
     void focusObjectChanged(QObject *focusObject);
+    void focusWindowChanged(QWindow *focusWindow);
 
 protected:
     bool event(QEvent *);