WET theme: Added mega menu and mobile menu support

Also fixes timerpoke race condition that prevented multiple menus from being loaded (made clone of the timerpoke array to ensure that selectors weren't being removed from the array while trying to fire events).

@masterbee I moved more structural menu CSS to menu.scss core so you can probably remove more from the theme. Only a first cut so would be good to get agreement on what goes in menu.scss versus theme so there is consistency across themes (and no duplication).