Revert "QDBusArgument: remove useless op<< overloads"

This reverts commit 5f542f3c

,
since it breaks streaming of types derived from QList and
the docs state that this should be possible without providing
custom op<</>> for such types.

Task-number: QTBUG-53376
Change-Id: I2bde714ac384f2aed67ad30decea702fb79aef1b
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusargument.h

@@ -261,6 +261,35 @@ inline const QDBusArgument &operator>>(const QDBusArgument &arg, Container<T> &l
     return arg;
 }
 
+// QList specializations
+template<typename T>
+inline QDBusArgument &operator<<(QDBusArgument &arg, const QList<T> &list)
+{
+    int id = qMetaTypeId<T>();
+    arg.beginArray(id);
+    typename QList<T>::ConstIterator it = list.constBegin();
+    typename QList<T>::ConstIterator end = list.constEnd();
+    for ( ; it != end; ++it)
+        arg << *it;
+    arg.endArray();
+    return arg;
+}
+
+template<typename T>
+inline const QDBusArgument &operator>>(const QDBusArgument &arg, QList<T> &list)
+{
+    arg.beginArray();
+    list.clear();
+    while (!arg.atEnd()) {
+        T item;
+        arg >> item;
+        list.push_back(item);
+    }
+    arg.endArray();
+
+    return arg;
+}
+
 inline QDBusArgument &operator<<(QDBusArgument &arg, const QVariantList &list)
 {
     int id = qMetaTypeId<QDBusVariant>();
@@ -273,6 +302,7 @@ inline QDBusArgument &operator<<(QDBusArgument &arg, const QVariantList &list)
     return arg;
 }
 
+// QMap specializations
 template<typename Key, typename T>
 inline QDBusArgument &operator<<(QDBusArgument &arg, const QMap<Key, T> &map)
 {
@@ -321,6 +351,7 @@ inline QDBusArgument &operator<<(QDBusArgument &arg, const QVariantMap &map)
     return arg;
 }
 
+// QHash specializations
 template<typename Key, typename T>
 inline QDBusArgument &operator<<(QDBusArgument &arg, const QHash<Key, T> &map)
 {