Fix offsetWidth and scroll bindings #13

Found four existing bugs while checking the Spago migration. They reproduce on both the original code and the migrated code; the migration does not change these bindings.

• HTMLElement.offsetWidth reads offsetWodth, so it returns undefined. A 180px-wide element with a 3px border on each side should return 186.
• Window.scroll calls scrollBy instead of scroll.
• Window.scrollTo also calls scrollBy. For both window methods, start at (31, 67), then request (13, 29) with instant scrolling. They move to (44, 96) instead of (13, 29).
• Element.scroll calls scrollIntoView instead of scroll. On an overflowing element starting at (43, 79), requesting (13, 29) leaves its own scroll position at (43, 79).

The bindings are in HTMLElement.js, Window.js, and Element.js.

Checked in Chromium using the compiled PureScript modules. For the scroll cases, use a page/element large enough to scroll in both directions and start at nonzero coordinates. Starting at zero hides the window bug. Firefox and Safari have not been checked.