Add numbering of rows and/or sentences to the editor

[cajosc]

• Useful when discussing issues.

[arildm]

Line numbering is probably not a good idea, because the text area width adjusts to the web browser width, so lines will wrapping is variable.

Sentence numbering is more reasonable. One caveat is that sentence segmentation may be edited in normalization. One sentence in source may correspond to three sentences in target. Source sentence 16 may correspond to target sentence 14. But that is probably fine.