plugin XEP-0166, XEP-0167, XEP-0234, XEP-0353: call events management to prepare for UI: - XEP-0166: add `jingle_preflight` and `jingle_preflight_cancel` methods to prepare a jingle session, principally used by XEP-0353 to create and cancel a session - XEP-0167: preflight methods implementation, workflow split in more methods/signals to handle UI and call events (e.g.: retract or reject a call) - XEP-0234: implementation of preflight methods as they are now mandatory - XEP-0353: handle various events using the new preflight methods rel 423