Add "total conversation" to the EN 301 549 Terms (clause 3)

There should be clearer reference to total conversation in EN 301 549, and this necessitates that we have a definition of it.

The following definition is the one present in the draft of ES 204 009 "Human Factors (HF); Requirements for (Interoperable) Total Conversation services":

total conversation: multimedia real time conversation service that provides bidirectional symmetric real time transfer of motion video, Real-Time Text and voice between users in two or more locations