This makes the `Resource` class really light-weight and a bit easier to create.
OTOH, there's no more the possibility to have convenience methods such as `name`, `shortname`, `url`...
Using their replacement functions `get_name`, `get_shortname`, `get_url` is therefore mandatory now, and slightly more verbose as we need to pass the env as first parameter. The corresponding helper functions in the template files of course don't need that `env` parameter.
