Add FileCollection to FileTransfer interface
Add FileCollection to FileTransfer interface

The implementation in Gabble hasn't changed since 2010, so it's
de facto stable. We should either spec it or delete it.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=26609
Reviewed-by: Guillaume Desmottes <f16649c7afcd84e3350287183c4ed23cb1aea57c@collabora.co.uk>

freedesktop-unofficial-mirror/telepathy__telepathy-spec,TelepathyIM/telepathy-spec