Raise exception in Java wrapper if issue in maybenil or notnil tag in doxygen header + more changes in methods' header

More work on documentation + modified JAVA wrapper to ensure all returned arrays are never null even if the C API says maybenil

Created linphone_core_search_chat_room to replace all linphone_core_get_chat_room and linphone_core_find_chat_room methods
Created linphone_core_create_chat_room_6 to replace all previous chat room creation methods

Add an existing root header to PUBLIC_HEADER property in order to generate a link to Headers folder on cocoapod


(cherry picked from commit cfaf00e3)

Revert changes regarding chat_message_sent callback notification (notified before message is really sent but right after chat_message_send() is called) + fixed crash

no longer issue presence subscription when only_when_registered = true but no proxy config configured.