QObjectCompatProperty: Reintroduce operator=

This is a partial revert of a1a2d97e34.
Reason: The new design does not notify automatically anymore, so using
operator= is safe to use.

Change-Id: I6cb735e40c0da72d22fcc426423eb7830901e5f4
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qproperty_p.h

@@ -471,6 +471,12 @@ public:
         this->val = t;
     }
 
+    QObjectCompatProperty &operator=(parameter_type newValue)
+    {
+        setValue(newValue);
+        return *this;
+    }
+
     QPropertyBinding<T> setBinding(const QPropertyBinding<T> &newBinding)
     {
         QtPrivate::QPropertyBindingData *bd = qGetBindingStorage(owner())->bindingData(this, true);