tabpane: use data- attribute instead of class name

[chalin]

• Contributes to Tabpane shortcode fixes and enhancements (July) #1605
• Switches to attaching the persistence key to tab HTML elements as a data- attribute rather than a CSS class name
• Preserves converting the persistence key to lowercase, though now there's no longer a new to replace any characters
• Other improvements to the JS to make it more robust

[deining]

LGTM