Screenshot link for units (API)

Hello everybody,

I use the Weblate API for the external recording app. In the project, sometimes Voice Over artist wants to see the screenshot before the recording. I am already using the "Screenshot feature" in the Weblate. But we can not take the screenshots links special for the units via API. So, it can be nice to see a `screenshot_link` field for these endpoints;

```/api/translations/PROJECT/COMPONENT/LANGUAGE/units/```
```/api/units/UNIT_ID```

Thanks!