Bug 41916: Letterboxing preferences UI

browser/components/preferences/jar.mn

browser/components/preferences/letterboxing-middle-dark.svg

browser/components/preferences/letterboxing-middle-light.svg

browser/components/preferences/letterboxing-top-dark.svg

browser/components/preferences/letterboxing-top-light.svg

browser/components/preferences/letterboxing.css

browser/components/preferences/letterboxing.inc.xhtml

browser/components/preferences/letterboxing.js

browser/components/preferences/main.inc.xhtml

browser/components/preferences/main.js

browser/components/preferences/preferences.xhtml