Store latest version number in registry

Storing the latest version number in the registry has been requested by content creators, so that their installers can validate the user's version of Open Rails is compatible with the content

This will be stored per-user, next to the existing content folders storage, which installers already modify to add content

The version will be stored in a machine-readable format (so that it may be updated automatically) and human-readable text for display purposes

Discussion: http://www.elvastower.com/forums/index.php?/topic/35458-store-latest-version-number-in-registry/
Roadmap: https://trello.com/c/6ybEXrey/508-store-latest-version-number-in-registry