Add support for pulling / merging in dataset from other model

[avatar42]

For instance The Roboflow plate set https://public.roboflow.com/object-detection/license-plates-us-eu/3 which is labled with license-plate and vehicle. Without going through quickLabel just remap the mapping files for ID 0 and 1 to whatever the next numbers would be.

[avatar42]

If names are same (or altered to be) then should just change to correct ID.