Figure out tabindex/focus handling

[psalaets]

tabindex were set for all buttons as part of #19 (5f78b6bbc4f30c118d93314a9113bb930c9ac821, fa477ac7e91faa3e587926833a7c22bfa866da00).

This improves a11y but now we get the default outline on a focused nav button obscuring the art. Maybe find a less obtrusive way to indicate focus?

Reading

• http://a11yproject.com/posts/never-remove-css-outlines/
• https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#tabindex-usage

[psalaets]

I've reverted the two commits above for now.