XAR format version check

mvn xar:verify now fails a page whose declared XAR format version (the version attribute of xwikidoc) is:

• invalid
• higher than 1.7, the highest one known by the targeted XWiki version
• lower than the one introducing an element the page uses:
  • an attachment's revisions: 1.4
  • originalMetadataAuthor: 1.5
  • enforceRequiredRights: 1.6
  • the type attribute of an object property: 1.7

mvn xar:format raises a version that is too low to the required one; an invalid version or one above 1.7 must be fixed by hand. See the XAR Maven plugin documentation.