Presence is not requested after accepting subscription

[daniele-athome]

Details at df4963b:

When accepting an invitation, the if below could be false because of delay or no reloading of mConversation at all thus skipping the presence request.
An automatic solution could be to emit a ROSTER_LOADED whenever the roster changes still this if should go away in favour of the message center checking for subscription status and the UI reacting to a pending request status (i.e. "pending_in")