Use observer instead of scroll events about cpu-15

[dascritch]

cf https://tech.mozfr.org/post/2017/08/21/IntersectionObserver-arrive-dans-Firefox
polyfill https://github.com/WICG/IntersectionObserver/tree/gh-pages/polyfill because Safari and iOS are deep shits

[dascritch]

Reverted 59a0cd6 , because observers on webcomponents seems not correctly checked in Firefox

Snippet for checking :

function check(elements) {
          console.log(elements[0].intersectionRatio, elements[0].isIntersecting, elements[0])
}
new IntersectionObserver(check).observe(document.getElementById('menu'));
new IntersectionObserver(check).observe(document.querySelector('cpu-audio'));

[dascritch]

Reported to Mozilla https://bugzilla.mozilla.org/show_bug.cgi?id=1581876

[dascritch]

Bug related to cpu-audio webcomponent : container not declared as block