Fix bug in designer support reference counting

The QtQuick designer support may override the meta-object and reparent the
property-cache of the object under editing. The code for replacing the parent
of the property cache however was not handling the refcount of the _parent
correctly.

Change-Id: Ic73294fc208b297e8ec9c0b775b5da01a309dba6
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>

src/qml/qml/qqmlpropertycache.cpp

@@ -452,7 +452,12 @@ QQmlPropertyCache *QQmlPropertyCache::parent() const
 
 void QQmlPropertyCache::setParent(QQmlPropertyCache *newParent)
 {
+    if (_parent == newParent)
+        return;
+    if (_parent)
+        _parent->release();
     _parent = newParent;
+    _parent->addref();
 }
 
 // Returns the first C++ type's QMetaObject - that is, the first QMetaObject not created by