QProxyStyle: do not pass DeferredDelete to baseStyle

Calling deleteLater on an object of type QProxyStyle (or inheriting from
QProxyStyle) results in this object never deleted, since it simply
passes the event to its base style, which in case of deleteLater is
not right. QProxyStyle inherits QCommonStyle which in turn inherits
QStyle (which is QObject's descendent). So for the style to be deleted
we pass DeferredDelete to its base class - QCommonStyle::event (which
means QObject::event, since neither QCommonStyle nor QStyle override
QObject::event()).

Pick-to: 6.3
Fixes: QTBUG-96213
Change-Id: I99b8f413624e2f18ddae3fb331997f767de219d0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/styles/qproxystyle.cpp

@@ -383,6 +383,10 @@ void QProxyStyle::unpolish(QApplication *app)
 bool QProxyStyle::event(QEvent *e)
 {
     Q_D (QProxyStyle);
+
+    if (e->type() == QEvent::DeferredDelete)
+        return QCommonStyle::event(e);
+
     d->ensureBaseStyle();
     return d->baseStyle->event(e);
 }