dule Polymer Core Elements

`core-menu-button` is a `core-icon-button` that opens a drop-down menu
that allows the user to select an option. You can position the drop-down
menu with the `halign` and `valign` properties, or use CSS if more control
over positioning is desired.

Example:

    <core-menu-button id="btn">
         <core-item icon="settings" label="Settings"></core-item>
         <core-item icon="dialog" label="Dialog"></core-item>
         <core-item icon="search" label="Search"></core-item>
   