provide application preference to enable or disable edits to invalid files

[dericed]

Related to #93, I suggest we set an application preference option of "Allow changes to invalid files" with a default to false.

Suggested behavior:
If "Allow changes to invalid files" = false, then
If someone tries to save changes to a list of files that includes an invalid file, I suggest that changes for valid files are saved, but an error message "Changes not saved to $filename because this file is invalid. If you want to force a change to this file, please enable Allow changes to invalid files, but be aware that edits to invalid files may have unexpected results or remove the indication that the file is invalid."