AssetAPI: New transfer prioritization functionality. This lets a implementation an change to prioritize the transfers before they are executed by the providers. Earlier there was no control over the order. Changes IAssetProvider a bit, in splitting creation and actual execution of the transfer. Default AssetAPI prioritization requests meshes before materials, the impl can be overridden from any 3rd party code to do fancier things. We should implement distance to camera based prioritization once the EntityWeakPtr data is available in transfers.