Tracking issue for developing a generic binding protocol to enable a set inter-element behaviors in AMP

Potential behaviors:

• Carousel thumbnails update main carousel index (and visa-versa)
• Tabbed UI
• Image thumbnails update main image display
• Toggle video src
• Form field dependencies
• Smart buttons
• Element tap expands non-contiguous <amp-accordion>
• Composible <amp-lightbox> groups
• Tooltips
• Trigger CSS class and value changes on scroll (sticky headers, animations on scroll)
• Trigger attribute change on window width
• Custom video controls

cc/ @rudygalfi @cramforce @dvoytenko @aghassemi