ibility

`<paper-menu>` has `role="menu"` by default. A multi-select menu will also have
`aria-multiselectable` set. It implements key bindings to navigate through the menu with the up and
down arrow keys, esc to exit the menu, and enter to activate a menu item. Typing the first letter
of a menu item will also focus it.

@group Paper Elements
@element paper-menu
@hero hero.svg
@demo demo/index.html
-->

<dom-module id="paper-menu">
  <template>
    <style include="paper-menu-shared-styles"></style>
    <sty