QDataWidgetMapper: micro-optimize clearMapping()

QList<WidgetMapper>::takeLast() is a needlessly expensive operation
(involves copying a QPersistentModelIndex and a QPointer).

Instead of looping over takeLast() until empty, reverse-iterate
over the list and call removeEventFilter(), then clear the whole
list.

We reverse-iterate to preserve existing behavior. Also, since the
original code popped the WidgetMapper instance before calling
removeEventFilter(), we move the whole list into a local copy
before iterating. There's little chance that a removeEventFilter()
call will cause reentrancy into QDataWidgetMapper, but better safe
than sorry.

This and the previous change together save 448 bytes of text size in
libQt5Widgets.

Change-Id: I1bfe907751659f31e618aa05bbb7b840f0aa61f4
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/itemviews/qdatawidgetmapper.cpp

@@ -762,10 +762,11 @@ void QDataWidgetMapper::clearMapping()
 {
     Q_D(QDataWidgetMapper);
 
-    while (!d->widgetMap.isEmpty()) {
-        QWidget *w = d->widgetMap.takeLast().widget;
-        if (w)
-            w->removeEventFilter(d->delegate);
+    QList<QDataWidgetMapperPrivate::WidgetMapper> copy;
+    d->widgetMap.swap(copy); // a C++98 move
+    for (std::reverse_iterator<QList<QDataWidgetMapperPrivate::WidgetMapper>::const_iterator> it(copy.cend()), end(copy.cbegin()); it != end; ++it) {
+        if (it->widget)
+            it->widget->removeEventFilter(d->delegate);
     }
 }