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.
filetransfer: rename sipe_core_ft_allocate()

Renamed to sipe_core_ft_create_outgoing.
Add media application type
mime: take Content-Transfer-Encoding into account

Lync file transfer sends file metadata in XML format encoded as
quoted-printable. This has to be decoded prior processing.

Implemented for both libpurple and GMime backends.
purple: remove unused media stream parameters

"transport-protocols" and "demultiplex-func" are no longer used.
filetransfer: parse incoming Lync-style filetransfer invite
filetransfer: initialize media session for incoming data
filetransfer: send downloadFile request after establishing a connection
media: add extra SDP for file transfer/desktop sharing
media: add sipe_data_session_new()

Creating sipe_media_call_private using this function won't display
the usual Pidgin dialog for accepting/declining the call, and neither
the call-in-progress window with audio level meters and video will
show up. This will be used for Lync file transfer, where we want the
user to interact with our media session in a different way.
filetransfer: initialize backend structure for Lync FT
media: add candidate_pair_established_cb to sipe_media_call
filetransfer: send downloadFile request after establishing a connection