[API] re-implemented file transfer

Saros is now using XMPP file transfer instead of custom made stuff.

Benefits: you can now send files regarding of the session status and
also to other XMPP clients.

TODO: there might be still some (UX) issues as the Smack FileTransfer
API is bugged as hell.

Change-Id: I89ee4df766586d5f17d723586b34ee938f0cf63b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1335
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>