How should we handle a forward slash "/" in new file and rename dialogs?

[Emmanuel Pescosta]

Hello mark,

in my opinion we should forbid slashes in filenames. I think this is
the best solution ;)
For example: Nautilus gives an error message "Slashes are not allowed
in filenames"

Btw. I'm already working on that: https://git.reviewboard.kde.org/r/107139/

Best regards
Emmanuel