Remember successfully copied files

[paulmueller]

There should be a disk-cache of files that have successfully been copied to the target directory.

• this should be located somewhere in the user's cache directory
• this should be part of the file verification and should come before MD5sum checks
• This could simply be a hash of the raw and target file name
• It should be possible to clear this cache (e.g. via CLI clear-cache)

[paulmueller]

The rationale behind this is that files that were copied once are "trusted" to not degrade in the target path. But one could also use the cache as a "failsafe" to check whether files in the input are rotten. Maybe one could also add a "verify" button that verifies files in input and output.