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
media: mux RTP and RTCP when using TCP connection

Lync uses a single TCP connection for both components.
media: duplicate TCP candidates
filetransfer: use 'init' callback in sipe_file_transfer

Replaces sipe_core_ft_incoming_init() and sipe_core_ft_outgoing_init()
and the specific callback method is now set by Sipe core depending on
whether incoming or outgoing file transfer is being handled.

Will allow for some code to be shared between legacy OCS file transfer
implementation and the new Lync approach.
filetransfer: use 'user_rejected' callback in sipe_file_transfer

Replaces sipe_core_ft_cancel(); also doesn't make sense to use it in
outbound file transfer.
filetransfer: use 'deallocate' callback in sipe_file_transfer

Replaces sipe_core_ft_deallocate().
filetransfer: use 'start' callback in sipe_file_transfer

Replaces sipe_core_tftp_incoming_start() and
sipe_core_tftp_outgoing_start().
filetransfer: use 'end' callback in sipe_file_transfer

Replaces sipe_core_tftp_incoming_stop() and
sipe_core_tftp_outgoing_stop().
filetransfer: use 'read' callback in sipe_file_transfer

Replaces sipe_core_tftp_read().
filetransfer: use 'write' callback in sipe_file_transfer

Replaces sipe_core_tftp_write().
filetransfer: move common PurpleXfer init into create_xfer()
filetransfer: remove sipe_purple_ft_new_xfer()

Not actually used by libpurple.