Switch to openttd-protocol

[TrueBrain]

Currently this repository implements its own protocol handler. There is a repository (https://github.com/OpenTTD/py-protocol) that unifies this between all our Python components.

Please switch to using that. It mostly means moving the openttd folder to that library, and remove what is not needed.

This also allows access to enums, meaning we can remove hard-coded values (and their comments) and replace them with the correct enum.