Add opportunity for keyboard navigation (a11y feature)

[toptalo]

I think that it would be good to have opportunity to navigate between day/time items in calendar with keyboard tab key and select target with space or enter key (if target is not .xdsoft_disabled ).

What do you think?

If agree I can try to make PR with this feature.

[zerolab]

I think this would be an immense and welcome improvement. We hit the accessibility issue with the picker in Wagtail - https://github.com/wagtail/wagtail/issues/5325