Implement layered translation repos support

[LeadRDRK]

Allow users to specify an extra layered translation repo. When checking for new updates, the updater will fetch the index for both repos and merge the updated files list together, with the main translation repo taking precedence - if a file exists in both the main repo and the layered repo, the one from the main repo will be preferred.

TODO: plans to have an option to prefer the layered repo instead, for patch reviews