Allow to know in ReleaseClass if a release has been done or not

ReleaseClass xclass provides a release date, but in practice we see that this information is not enough to know if a release has been done or not: we often create a release plan in XWiki.org with a release date that happens to be wrong because we need to delay the release. So sometimes the release is not done, even if the date is passed.

To be able to really rely on this xclass to know if a release is done or not, it would be good to add a boolean property that we could set to true when the release is actually done.