Add support for file downloads

[nijel]

Moved from https://github.com/nijel/weblate/issues/1293

We would like to establish a workflow where we have more control over the time when translations are pulled into the main project and would find it handy to use wlc to download the files.

Right now, we use a seperate git repo that is included through a submodule, but submodules always need a bit more attention than a simple folder would.

How are the ideas on implementing this?

[nijel]

There is API on the server for this, it's just not implemented in wlc:

https://docs.weblate.org/en/latest/api.html#get--api-translations-(string-project)-(string-component)-(string-language)-file-