Added new method to the OtrEngineListener interface.

Hello,

- Added a new method to the OtrEngineListener interface. Signature: sessionIsStarting(SessionID session);
in order for applications to react when a new session is about to start or more precisely, when OtrEngine.startSession(). The reaction for example can be sending a message to the user that initiating private conversation is in progress, or in Jitsi's case - change the padlock icon to "loading padlock"