As part of #1532 it was identified that long running file transfers
could get lost deep in the chatlog. This could result in unexpected use
of bandwidth over time if users lose track of old/large transfers. This
commit updates the files form to show in progress file transfers and
offer a way to control them.
* FilesForm now works on ToxFiles instead of finished file paths
* FilesForm widgets have been replaced with an MV tree view with depth
1. The existing QListWidget did not provide us the controls to render
more complex items. The use of delegates allows us to efficiently draw
progress bars and controls
* getHumanReadableSize has been extracted from FileTransferWidget into a
more general utils file
