xcb: eradicate Q_FOREACH loops [const-& returns]

... by replacing them with C++11 range-for loops.

The function QObject::children() returns by const-reference,
so its result can be passed to range-for without further changes.

Saves ~300B in text size on optimized GCC 6.0 Linux AMD64 builds.

Change-Id: I8360e946774b0d30233c0fa68f318872da61e867
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -286,7 +286,7 @@ static inline XTextProperty* qstringToXTP(Display *dpy, const QString& s)
 // TODO move this into a utility function in QWindow or QGuiApplication
 static QWindow *childWindowAt(QWindow *win, const QPoint &p)
 {
-    foreach (QObject *obj, win->children()) {
+    for (QObject *obj : win->children()) {
         if (obj->isWindowType()) {
             QWindow *childWin = static_cast<QWindow *>(obj);
             if (childWin->isVisible()) {