dropped all FT* structures and code, it was copy-pasted from Spectrum1
and never worked.
added Attachment structure to the protocol, ConversationMessage now
able to have urls attached.
XMPP frontend maps Attachment to jabber:x:oob payload.
Dropped Xfer UI from the libpurple backend, we only need to handle
file-recv-request signal to accept file transfer and file-recv-complete
signal to forward received file. Files are received in the web.directory.
Fixes #367