title: On Drop target enter and leave custom event
body: <p>I have one drop target div which have some nested div's, i add dragover event to parent div and add 'drag-allow' attrib to nested divs. I want to add custom event fires when mouse enters or leave any child div.</p>
  
  <pre><code>&lt;div id="container"&gt;
     &lt;div id="page1" drag-allow &gt;
      Page #1
     &lt;/div&gt;
     &lt;div id="page2" drag-allow&gt;
      Page #2
     &lt;/div&gt;
     &lt;div id="page3" drag-allow&gt;
      Page #3
     &lt;/div&gt;
  </code></pre>
  
  <p></p>
  
  <pre><code>    var ground = document.getElementById('container');
  ...
  ground.addEventListener('dragover', function(e){
      evt.preventDefault();
      evt.stopPropagation();
  
      evt.target.id; // custom event will fire based on child div's id
  
  }, false);
  </code></pre>