New Stateless File Sharing version with backwards compatibility

Update XEP-0447: Stateless file sharing to v0.3.1.

QXmpp:

• Implement new fallback element
  https://github.com/qxmpp-project/qxmpp/pull/608
• Add IDs to QXmppFileShare, Add attached sources to QXmppMessage
  https://github.com/qxmpp-project/qxmpp/pull/613

Kaidan (!1119 (merged)):

• DB: Add new file IDs from XEP
• Implement attaching of new sources to files
• Send extra fallback messages for old clients

Fixes/adjustments to the XEP:

• XEP-0428: Change schema int types to xs:unsignedInt
  https://github.com/xsf/xeps/pull/1335