Enhanced backup progress indication

[KraXen72]

The problem

When running a long backup, it's a bit hard to tell how far it's progressed and estimae how long it's to being done.
The current report of 0.3TB and the currently backed up file is nice since it shows, that the backup is still running and working, but I think it could be better.

Requested Solution

I suggest the following changes:

• Show the Original and Deduplicated sizes in GB or GiB (whichever you're using throughout the app), up to 100TB.
  • I think most people are doing backups < 10TB, when it's much more helpful then to then see 7848GB instead of 7.8TB. Even above 10TB, 10 748GB is still quite readable imo. Especially for backups > 100 GB and < 1TB, it's not very helpful to see 0.3TB.
• Display the Source folder total size, e.g. Original: 340 GB / 780 GB, Deduplicated: 300GB
  • Start crawling the source folder when a backup is started, to see it's full size. Until the crawl is finished, only display something like: Original: 340 GB, Deduplicated: 300GB. However, once the crawl is done, you can update it to Original: 340 GB / 780 GB, Deduplicated: 300GB. This is what Kopia does and I've found it quite helpful.
• Display the average copy/backup speed over the last x seconds
  • This one's not as important but it also help give the user some more data. Similar to how you'd see the avg speed over a certain period in a file copy dialog, I think seeing something like 23 MB/s could be quite nice.
• Introduce a progress bar
  • This one is optional as it might not be desired, but once you have the current processed files and the total size of the soruce folder, you could display a progress bar showing how far along the backup is in terms of processing the Source folder. There's no need to display an ETA!

Alternatives

Keeping the current system.

Additional context

I have read #594 and I understand that estimating an ETA can be considered foolish, so I'm not suggesting that. I am merely suggesting to surface more information to the user, so they can have a better idea how the backup process is going.