Menu bar: Changed structure of menu include to be more appropriate for WAI-ARIA and hard-coded in WAI-ARIA attributes

Notes:
1. New structure messes up CSS a bit. Now pushes menu down (since sub-menus are now descendents of the menu bar items).
2. Keyboard didn't work even before this change. Seems to be because the menu closes when focus is being changed to a sub-menu item.
3. I hardcoded the WAI-ARIA attributes for now. Some may make sense to have hard-coded in the markup by default but others will have to be made dynamic (in particular the ones that change such as aria-hidden and aria-expanded). Probably also good to make dynamic the setsize and posinset (otherwise risk of human error and can be onerous).
4. Menu is missing the hoverintent-style delay. We got rid of hoverintent in v3.1 and replaced with custom code but the 0.5 second delay is still required (for mouse hover).
5. Not sure how this is supposed to render on small screens.
6. Removed the extra h1 and unnecessary section element in the working example. Also changed Navigation to Menu.

/cc @masterbee