Use observer instead of scroll events

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cf https://hacks.mozilla.org/2017/08/intersection-observer-comes-to-firefox/

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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'));

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

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