Update to RFC6120/RFC6121

[lnjX]

I found the following tasks to fully comply with RFC6120/RFC6121 by reading the 'Differences from RFC3920/RFC3921' appendices. (I didn't read through the whole RFCs, possible features that missed before are not included)

RFC6120

• QXmppStanza::Error:
  • Remove/Deprecate payment-required condition (PR Deprecate PaymentRequired error condition #277)
  • Add missing policy-violation condition (PR Add PolicyViolation stanza error condition #279)
  • Add parsing for optional redirect URI for gone and redirect conditions (PR QXmppStanza::Error: Add redirection URI from RFC6120 #280)
  • Add new optional by attribute (PR QXmppStanza::Error: Add 'by' attribute from RFC6120 #283)

RFC6121

• Optional Roster Versioning
  • Add manager support (Issue Support Roster Versioning from RFC6121 #164)
  • Add stream feature
• Pre-approved presence subscription (PR Pre-approved subscription requests #285)
  • stream feature
  • roster item approved attribute
  • rostermanager?
• Add parent attribute for <thread/> (PR QXmppMessage: Add parent thread attribute from RFC6121 #284)

[lnjX]

Since roster versioning is only optional, we can close this issue already. (That doesn't mean I don't want to implement it)