Verify files after transfer without using a checksum

Since we have to read both the source (again) and destination files in order to verify them, there's no reason to calculate a checksum: we can simply compare the file contents with each other.

Even faster might be to stop using g_file_copy(), and instead "manually" copy the file so we can feed the source bytes into a checksum generator on the fly, without having to re-read the source file again at the end. We could even chunk the file and generate a checksum per chunk in parallel on different threads, and then compare the chunk checksums at the end.

But this should give us a nice speedup, and is much, much simpler.

Closes #1471 (closed)