Contribution Version Number, Latest Contribution Version available and Last Date on which contribution was updated now show (Resolves #2498 and #2561)

This PR has commits to resolve [#2498](https://github.com/processing/processing/issues/2498) and [#2561](https://github.com/processing/processing/issues/2561).

Note that a lastUpdated field will be required in the contributions.txt and the .properties files. The field will have to be a Unix Timestamp in milliseconds from January 1st, 1970 at UTC. However, the transition can be gradual, as the code ensures that no errors occur even when these fields are not found.