[Expression builder] show represented values in preview

[m-kuhn]

Instead of only showing the raw values, show also the represented values in the expression builder in the list values preview.
This helps people to choose from readable values (whereas still the raw values get inserted on double click).

Should we ship this as a UX fix or wait for post 3.8? Kind of on the boarder between feature and bugfix

[m-kuhn]

@nyalldawson and anyone else, what's your opinion:

• include in 3.8?
• backport to 3.4?

I think it could be treated as a UX fix (merge to 3.8 now, wait for 1-2 weeks of field testing before merging the backport

[stale]

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[luipir]

merge to 3.8 now, wait for 1-2 weeks of field testing before merging the backport

it's safe for me

[m-kuhn]

I'll read this as an approval of this PR, thanks @luipir ;)