Update to XEP-0420 version 0.4.0 and adapt namespace to shortname:

* Replace SCE's old 'content' element by its new 'envelope' element
* Replace SCE's old 'payload' element by its new 'content' element
* Update SCE's namespace to 'urn:xmpp:sce:1'
* Change namespace to 'urn:xmpp:tm:0'