JingleSession: only emit about-to-initiate once
JingleSession: only emit about-to-initiate once

Previously, about-to-initiate would be emitted on every call to
try_session_initiate_or_accept(), whether or not all contents were
actually ready.

This is relatively harmless – it's only used as a hook for sending
directed presence to contacts not subscribed to us – but looked wrong
and is redundant.

noonien-d/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky