Frontend plugin integration scenario

The new API could allow a scenario I'd like to propose, which is based on https://www.transifex.com/lp/transifex-live/
- [x] Authorization via configurable API key per project - #4974
- [x] Authorization via configurable API key per user
- [x] Read all translations from a component
- [x] Read all translations from a component in a specific language
- [ ] Read one translation from a component in all translated languages
- [x] Write a translation (
- [ ] Write a comment with a translation
- [ ] Write a comment with an image of the translation (could use https://experiments.hertzen.com/jsfeedback/ which uses https://github.com/niklasvh/html2canvas to create image and highlight)
- [x] Get component translation process (for status updates)
- [x] Get repository status (in case upstream branch changed)
- [x] Push changes and get response (if API key has right authorizations)

Also see what a frontend library would look like: https://github.com/transifex/transifex-live-wordpress

This would be a really big step in continuous translation, from a translator's perspective. Please let me know what you think, maybe the API already has most use cases covered?
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