Rework presence to remove online and offline virtual activities and introduce consolidated presence.

Denying a friend to subscribe presence was no more possible
once we accepted.

Fix bug 0001391: An incorrect presence activity is added if not is contained in a received presence XML.

No longer rely on the uri field of LinphoneFriend being set (use linphone_friend_get_address() instead).

Handle versioning of presence list notifications + store information that we have received presence information for a friend.