What does this PR change? What problem does it solve?

This PR replaces restic.Progress with a new type, ui/progress.Counter. It fixes several issues:

• Progress.Done races with Progress.reporter for access to Progress.fnM. As a result, a progress report can be printed after the Progress has been canceled.
• Progress.Done does not wait for termination, so it races with other routines for stdout, or the final message may not be printed before the process exits.
• Initialization is spread out over the constructor and various init functions, that are called even for processes that do not use a Progress.
• Progress is defined in internal/restic, overloading the meaning of that package.
• Callbacks may be called either from a separate goroutine or from Progress.Report's caller's goroutine. This was probably meant as an optimization, but it means that the caller may have to wait on stdout.
• Progress is designed to handle restic.Stat objects, which carry counts of various object types. Its callers (ab)use only its blobs count for various purposes, most of which are not counting blobs.
• The code has grown organically over time, making it complicated and leaving dead code in various places.

Instead of fixing these issues one by one, I've decided to write a replacement from scratch. It's >100 lines shorter, leaving ample space for a unit test that covers 92% of its statements.

Was the change discussed in an issue or in the forum before?

No.

Checklist

• I have read the Contribution Guidelines
• I have enabled maintainer edits for this PR
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review