title: Set selected item in paper-drop-down-menu?
body: <p>I have this paper-dropdown-menu with one button:</p>
  
  <pre><code>&lt;paper-dropdown-menu id="Default" label="My items"&gt;
    &lt;paper-menu  id="DefaultDropDown" attr-for-selected="value" selected="{{selection}}" class="dropdown-content"&gt;
      &lt;paper-item value="5"&gt;Item 1&lt;/paper-item&gt;
      &lt;paper-item value="6"&gt;Item 2&lt;/paper-item&gt;
      &lt;paper-item value="7"&gt;Item 3&lt;/paper-item&gt;
    &lt;/paper-menu&gt;
  &lt;/paper-dropdown-menu&gt;
  &lt;button onclick="clickEvent();"&gt;Click&lt;/button&gt;
  
  &lt;script&gt;
    function clickEvent() {
      $('#Default').selectedItem = -1;
    }
  &lt;/script&gt;
  </code></pre>
  
  <p>I want in the event "clickEvent" you can set selection dropdown-menu to -1: noT selected item.</p>
  
  <p>Any idea how?</p>
  
  <p>The code I put javascript is only to illustrate the idea</p>