Add lanuguageicons to Widget values

Problem/Motivation

The formatter supports languageicons.
The widget does not, yet. It should, for better UX

Steps to reproduce

- open the formatter settings, there is an option for the icon
- open the widget settings, there is no option for the icon

Proposed resolution

Comment from @stefankorn:

Maybe like this: https://developer.mozilla.org/en-US/docs/Learn_web_development/Extension...

Or maybe using Unicode emoji flags, like so: https://gist.github.com/chrisjonesio/2f693fd9853c5450345e18c836577393

But that surely is a bigger feature request. The Unicode emoji flags seem like an interesting thing more generally. Like having a service or library that returns you the flag emoji if you pass ISO 3166 code or something like that.

Remaining tasks

User interface changes

API changes

Data model changes