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.
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: Need to unescape the xml before parsing it
The xml we receive has "=3D" instead of "=" so attributes can't get
parsed, and it has "=\n" for line continuation it seems.
WIP: duplicate RTP candidates, and add extra SDP for ft/desktop sharing