Getting file path for external tile sets

[DidierMalenfant]

I'm feeding the parsed level to a build dependency tool and it would be useful for me to get access to the full file paths for external tile sets so that the built level can depend on those too.

Having looked at the code, and unless I'm mistaken, I don't think that this is accessible already. Would it be ok for me to put together a PR to add this?

Idea would be that when creating a tileset, pass the entire path to the external file instead of just the parent folder, store that and provide an access method to it.

Thanks!