Check if resource has been downloaded already

[gajop]

I think this relates to #77
We should be able to detect whether we already have a resource (map, game or engine), before initiating download. This should use local (downloaded) rapid tags. Another function should be used to obtain (refresh) newer rapid tags.

My use case for this is that I want to check with rapid if something exist, and in that case, I won't demand the user download the newer version (e.g. offline or no-update mode). However I still want to ensure the user has a version, since it's necessary for launching said resource.