checking against set of valid characters during GT production

[chreul]

especially when producing GT as a group it is sometimes hard to ensure consistency regarding transcription guidelines. therefore, it should be possible to define and exchange (json?) a set of "valid" characters. when users type an "invalid" character during ground truth production they should get some kind of hint that they are about to do something... unintended. details (type of notice, options for the users, ...) to be determined.

(should be compatible with the OCR4all check and regularize functionality planned within the OCR4all-libraries project.)
(probably something for after the vue.js transition. just a reminder for later)