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Improve documentation of linphone_core_pause_call(), linphone_core_resume_call() and linphone_core_update_call().

This reverts commit 16dadb5f.

* 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.