This means that we no longer rely on the remote end to close the
connection after sending the file, but additionally use the `<size>`
element from the initial file transfer `<description>` to check whether
the file transfer has been completed.
This was motivated by Conversations not closing the connection for
SOCKS5 file transfers.
This is still disabled by default until prioritization is implemented;
otherwise this could be preferred to HTTP uploads.
File sending only works via Jingle In-Band-Bytestreams right now, more
transports are going to be implemented.
To test this, uncomment the line with `JingleFileTransfer` in
libdino/src/application.vala.
