[Enhancement] Simple extra files manager

[ownedbycats]

https://www.mobileread.com/forums/showpost.php?p=4330641&postcount=16

The issue I had in this post happened because I had the extra data folder open at the time.

As extra data kind of breaks the 'stay out of the Calibre filesystem' rule, this makes me think it might make sense to have a dialog that shows a list of data files in a list similar to the 'view specific format' dialog. This would show the icon and filename.

Options would be:
- Add a new file
- Open the selected file. Use the 'open with' list if available for the extension; otherwise use system default.
- Rename a file.
- Delete a file.

Thanks in advance.

[ownedbycats (ownedbycats)]

I would also make this a separate toolbar button, perhaps with 'view extra data dialog' and 'open data folder' as its two options. (Though the latter may be unnecessary as it's already in the 'open folder' dialog.)

Putting it under 'view' is another option but may confuse the data-is-not-formats point.

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.