QUntypedBindable: Allow controlled access to internals

In declarative, we need direct access to the bindable's pointers. Enable
this via a internal only helper struct in qproperty_p.h.

Change-Id: I56e4622f570ff08f85f1161af608658ea510671e
Reviewed-by: Maximilian Goldstein <max.goldstein@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.h

@@ -566,6 +566,7 @@ Q_CORE_EXPORT void printMetaTypeMismatch(QMetaType actual, QMetaType expected);
 
 class QUntypedBindable
 {
+    friend struct QUntypedBindablePrivate; // allows access to internal data
 protected:
     QUntypedPropertyData *data = nullptr;
     const QtPrivate::QBindableInterface *iface = nullptr;

src/corelib/kernel/qproperty_p.h

@@ -629,6 +629,19 @@ Q_CORE_EXPORT BindingEvaluationState *suspendCurrentBindingStatus();
 Q_CORE_EXPORT void restoreBindingStatus(BindingEvaluationState *status);
 }
 
+struct QUntypedBindablePrivate
+{
+    static QtPrivate::QBindableInterface const *getInterface(const QUntypedBindable &bindable)
+    {
+        return bindable.iface;
+    }
+
+    static QUntypedPropertyData *getPropertyData(const QUntypedBindable &bindable)
+    {
+        return bindable.data;
+    }
+};
+
 QT_END_NAMESPACE
 
 #endif // QPROPERTY_P_H