QCompleter: Add check before setting the widget

This prevents unnecessarily removing and reinstalling the completer
as event filter on the same widget.

This does not prevent what's going on in QComboBox::focusInEvent(),
where we'd set the line edit as widget just to immediately override
it with combo box itself.

Change-Id: I70c081a920f4daf4d7560e5cd7158e4070042d42
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/util/qcompleter.cpp

@@ -1013,11 +1013,15 @@ QCompleter::~QCompleter()
 void QCompleter::setWidget(QWidget *widget)
 {
     Q_D(QCompleter);
+    if (widget == d->widget)
+        return;
+
     if (d->widget)
         d->widget->removeEventFilter(this);
     d->widget = widget;
     if (d->widget)
         d->widget->installEventFilter(this);
+
     if (d->popup) {
         d->popup->hide();
         d->popup->setFocusProxy(d->widget);