purple: fix build against 3.0.x API

Function purple_conversation_write_system_message() replaces
purple_conversation_write().

Renamed purple_message_get_who() -> purple_message_get_recipient().
srtp: parse encryption key from SDP
srtp: generate encryption key for each stream
srtp: offer encryption key when initiating a call
srtp: implement sipe_backend_media_set_encryption_keys()
notify: store ucPC2PCAVEncryption value form server
srtp: add sipe_backend_media_get_encryption_policy()
media: send encryption SDP attribute based on policy
media: add stream_decryption_keys list to sipe_media_private
media: refactor encryption compatibility check
srtp: negotiate encryption and apply keys to backend
srtp: encryption policies WIP