Toolbar buttons aren’t keyboard-focusable (tabindex="-1")

[joydicurtis]

Problem

Buttons in .nw-toolbar use tabindex="-1", so they’re skipped in the Tab order and can’t be operated with the keyboard.

Suggested fix

• Implement roving tabindex: exactly one button has tabindex="0", the others -1; update the focus on arrow keys.
• Provide a way to set an appropriate aria-label for each button.
• Mark decorative icons as aria-hidden="true" (and set focusable="false" on SVGs).

[stevermeister]

@joydicurtis #241 please review