menu-button: improve functionality

[leifoolsen]

Things to consider for improvement:

• on startup: move menu to document.body. This will fix z-index issues. Can use position absolute instead of fixed, meaning there is no need to close menu if document scrolls.. Drawback is that we have to delete the menu (or move it back to original parent) when the component downgrades (menu-button: move menu to document.body. #79). Completed.
• re-position menu on scroll, window resize or orientation change (menu-button: menu should re-position on scroll, window resize or orientation change #80). Completed.
• add menu items after page load.
• multi select, with check boxes and radio buttons and/or chips
• search filter, with callback
• preferred tether position
• provide some kind of tooltip if menu text overflows, e.g. use alt= or title=
• ripple effect
• animated open/close
• "richer" javascript api