Part of #4152

Implement the following UX where each slide starts with displaying description text in the bottom of the viewer and when user taps, the description fades away ( and comes back on the next tap )

@zhouyx Similar to the thumbnail work, we can have a getDescription(element) in the manager that would return (for now) just string text. Let's also use a similar logic as <amp-image-lightbox> on automatically finding the description for an element for now and later expand on it. Of course there may not be a description for an element at all which means Detailed View = Focused View in that case.

Let's also worry about long description text and if we can have scrolling in the description section. We can iterate on the UX but initially let's not allow the description overlay to go over 50% of the height.