Keyboard focus and arrows when modal is opened in Gallery

[samikeijonen]

It would be nice accessibility enhancement if these two would be implemented:

Next and previous arrows would be inside a button. They would have native support for keyboard accessibility. Markup suggestion:

<button class="featherlight-previous" arial-label="Previous image">SVG for arrow</button>

When using Tab or Shift+Tab keyboard focus should stay between previous, next, and close buttons inside the modal.

I can help with PR if needed.