Add support for XEP-0466: Ephemeral Messages
https://xmpp.org/extensions/xep-0466.html
This XEP allows to set a lifespan on Messages after that they shall be deleted. This would allow for certain use cases not yet possible. And improve the "Datensparsamkeit" of Gajim. So that if a Device gets in the Wrong hands Damage done by very Old Chathistory's is greatly reduced