Feature request: Add --keep-translation flag to merge command

[mrtryhard]

Enhancement's description:
Merge currently has this issue where you cannot just receive additional strings upon an existing translation.

It would be much better to add a flag that allows taking in only new messages and contexts without overriding the existing base messages / contexts.

Use case

• Translatable strings are extracted from software, producing base-new.ts
• An already, previous translation version exists japanese-old.ts
• You want to keep the already translated strings, but you might want to update the location, line number, comments, etc.

Expected behavior / output:

./qt-ts-tools merge japanese-old.ts base-new.ts --keep-translation -o japanese-to-translate.ts

The file japanese-to-translate.ts contains the new locations, messages, and context from the base-new.ts file, and keeps the already translated strings.

Sample translation file

To be refined

Sample expected / correct output file

To be refined

Question

• Should it be the default behaviour ? Should it be a flag to override existing translation ?

[mrtryhard]

Note
This ticket changes the merge behavior a little bit, notably:

• Order of merged messages and contexts (although it was not and is never promised to be sorted, you need to use sort for that)
• location nodes are now updated with the right file information.
• translator_comment is now updated

Feel free to report a bug if some behavior is questionable.