fix: make add column buttons focussable

[barmac]

Closes #847

[nikku]

While it is technically focusable now I'd consider to fine-tune the styling a little bit:

One way to do it to add an outline-offset: 4px;, resulting in this:

[barmac]

Indeed, the outline looks invisible on your screencast. In my browser (Chrome on Mac), it is easily distinguishable. I double-checked and it's also not visible without a magnifying glass o.O

[barmac]

I've added the change from your suggestion.

[barmac]

Thanks for checking.

[nikku]

Works great 🍰