Invalid characters in filenames suck.

Any plugins in FileSieve that return an invalid character results in the file the plugin is working on being skipped due to a failure when combining paths together. This is a controlled failure, I suppose you could say, but it's a failure nonetheless.

Despite adding an Invalid Filename Editor to the previous version of FileSieve, I disabled it as I wasn't sure at which step invalid characters should be substituted. Well, due to a quirk/bug involving filenames as reported by a user, I had another go at getting this working again.

Substitution now happens before path combining occurs, which I'm not 100% sure is the correct location. More of a semantic thing, but I'd like to move it to another step - not easy to do as this occurs during a (project-wide) plugin that does not reference the main, parent application.