Use the new options element instead of text fields for multi-value settings

[klonos]

In Backdrop 1.12, we have implemented an options element, which improves the UX when entering multiple values (backdrop/backdrop-issues#1005). We should use that instead of text fields.

One concern is backwards combatiility. We can require dependencies[] = backdrop:system (>=1.12) in the .info file of the module, but that means that we do not support previous versions of core. Is this OK?

[herbdool]

If you use a select field you get the new function automatically on newer BD versions. No need to declare dependencies.

I've only tried this with editing select fields. I suspect it doesn't work with the Form API.

[herbdool]

I'm going to close this since it's not possible.