GluonObject: Fix usage of addChildAt in combination with GameObject
GluonObject: Fix usage of addChildAt in combination with GameObject

REVIEW: 103054

This happened when calling addChildAt on a GameObject:
- GameObject adds the child to an internal list, d->children.
- GameObject calls GluonObject::addChildAt
- GluonObject calls addChild. As addChild is virtual,
  GameObject::addChild is called.
- GameObject checks whether the child is in d->children. As it is in it,
  it does nothing. GluonObject::addChild is not called.

I solved the problem by calling GluonObject::addChild explicitly
in GluonObject::addChildAt.

KDE/gluon,KDE/gluon,KDE/gluon,KDE/gluon