[FileTransferJob] Set file amount

Summary: This tells the new notification system that we're copying a file and lets it show a better summary.

Test Plan:

Is the composite job stuff already merged? That probably needs adjusting too

Reviewers: Plasma, nicolasfella

Reviewed By: nicolasfella

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D21182