Add option to screenshot a single post

### Describe the request It would be nice to have an option to get a nice-looking screenshot of a single post. The rational of this issue is similar to the feature many (all?) GTK's apps have, Tuba included, to get a screenshot of the app via hederbar --> right click --> Take Screenshot. You can always make a screenshot with the DE tool but it's not a single click behaviour and the result is not optimal. Use case for this feature request: Find an interesting post and want to share it on social media / whatsapp / mail. This follows the GNOME HIG, I guess. ### Checks - [x] This follows the [GNOME HIG](https://developer.gnome.org/hig/).