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

* Introduction of LinphoneFactory singleton class to create
  core-independent object like LinphoneAddress or LinphoneVcard.
* Make several C structures inherite from belle_sip_object_t class:
	* LinphoneCore
	* LinphoneVcard
	* LinphoneAuthInfo
* Creation of the LinphoneCoreCbs class that enable to store the
  callbacks used by LinphoneCore.

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).

Improve presence tester.
Add tester with ACk losses

This reverts commit e2609f96.

This reverts commit 9eca8962.

-unsubscribe in linphone_core_uninit 

Partially fix initial subscribe no longer working. Still need to rework friends file storage which is no longer available when compile with FRIENDS_SQL_STORAGE_ENABLED