Rework MultiCombo fields UI to accept a choice of values

[jolesh]

Diet tagging has three possible values: yes, no, only

Maybe make a small table with the diet forms on the left and the values selectable, like this:
Diets:
| vegetarian | no | yes | only |
| vegan | no | yes | only |
| halal | no | yes | only |
...

Maybe accessible through a menu, just like opening hours opens a dialog.

[Robot8A]

I support this. Checking the code, it should require either adding a new ComboType, or changing the current ComboType.multi behaviour.

[Zverik]

Hmm. It looks like I did the multiCombo fields wrong, not even supporting "no" values. The iD editor does it right though.

One question: can multiple diet options have the only value?

[Robot8A]

Yes, one restaurant could be both for example only gluten-free and only vegetarian.

[patepelo]

It would be similar to "Take away only" field.

Maybe we can take the code from there and reuse it?