Return a plain QMetaObject* from toDynamicMetaObject()

The only place where we use it, we swiftly cast it to QMetaObject*
anyway. Generating an object with virtual methods, only to then cast to
its non-virtual base type is dangerous. Also, if we are required to
return a QAbstractDynamicMetaObject from a method of
QDynamicMetaObjectData, we cannot implement QDynamicMetaObjectData
without also implementing QAbstractDynamicMetaObject. This indirectly
forces us to copy metaobjects around as we cannot wrap an existing
metaobject into a QDynamicMetaObjectData subclass.

Copying metaobjects around makes QMetaObject::inherits() unusable
because inherits() assumes identity is determined by identity of the
pointer.

Change-Id: Icbe697efa6fd66649eef3c91260c95cda62e3c90
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qobject.h

@@ -111,6 +111,9 @@ public:
     int postedEvents;
     QDynamicMetaObjectData *metaObject;
     QBindingStorage bindingStorage;
+
+    // ### Qt7: Make this return a const QMetaObject *. You should not mess with
+    //          the metaobjects of existing objects.
     QMetaObject *dynamicMetaObject() const;
 
 #ifdef QT_DEBUG

src/corelib/kernel/qobject_p.h

@@ -656,7 +656,7 @@ struct Q_CORE_EXPORT QDynamicMetaObjectData
     virtual ~QDynamicMetaObjectData();
     virtual void objectDestroyed(QObject *) { delete this; }
 
-    virtual QAbstractDynamicMetaObject *toDynamicMetaObject(QObject *) = 0;
+    virtual QMetaObject *toDynamicMetaObject(QObject *) = 0;
     virtual int metaCall(QObject *, QMetaObject::Call, int _id, void **) = 0;
 };
 
@@ -664,7 +664,7 @@ struct Q_CORE_EXPORT QAbstractDynamicMetaObject : public QDynamicMetaObjectData,
 {
     ~QAbstractDynamicMetaObject();
 
-    QAbstractDynamicMetaObject *toDynamicMetaObject(QObject *) override { return this; }
+    QMetaObject *toDynamicMetaObject(QObject *) override { return this; }
     virtual int createProperty(const char *, const char *) { return -1; }
     int metaCall(QObject *, QMetaObject::Call c, int _id, void **a) override
     { return metaCall(c, _id, a); }