core (xmpp): fixed workflow with post_xml_treatments in sendMessage: post_xml_treatments was incorrectly added to workflow using chainDeferred (which only add the callback, so post_xml_treatments own callbacks were not waited). As a result, plugins callback could not be waited as expected before a message was sent. This patch fixes it by not using chainDeferred anymore.

[D-BUS Service]
Name=org.salutatoi.SAT
Exec=sat