Add concept of deprecated translation keys

It would be nice to be able to mark keys as being deprecated.

For example on the XWiki open source project, we keep old deprecated keys in a special section in our Java properties file since we don't want to break extensions using those keys. OTOH on weblate we'd like them to be filterable/hidden by default or at least tagged as "deprecated" so that users know that they shouldn't focus on them and that they're not the important ones.

Possible? :)

Thx