title: How to add plus + and minus - button in quantity box of product listing page (list.phtml)
body: <p>Below is my code,</p>
  
  <pre><code>&lt;?php if($_product-&gt;isSaleable()): ?&gt;
  &lt;script type="text/javascript"&gt;
  function setQty(id, url) {
      var qty = document.getElementById('qty_' + id).value;
      document.getElementById('cart_button_' + id).innerHTML = '&lt;button type="button" class="button" onclick="setLocation(\'' + url + 'qty/' + qty + '/\')"&gt;&lt;span&gt;&lt;span&gt;Add to Cart&lt;/span&gt;&lt;/span&gt;&lt;/button&gt;';   
  }
  &lt;/script&gt;
  &lt;label for="qty"&gt;&lt;?php echo $this-&gt;__('Qty:') ?&gt;&lt;/label&gt;
  &lt;input type="text" name="qty_&lt;?php echo $_product-&gt;getId(); ?&gt;" id="qty_&lt;?php echo $_product-&gt;getId(); ?&gt;" maxlength="12" value="1" onkeyup="setQty(&lt;?php echo $_product-&gt;getId(); ?&gt;, '&lt;?php echo $this-&gt;getAddToCartUrl($_product) ?&gt;');" title="&lt;?php echo $this-&gt;__('Qty') ?&gt;" class="input-text qty" /&gt;
  &lt;span id="cart_button_&lt;?php echo $_product-&gt;getId(); ?&gt;"&gt;&lt;button type="button" class="button" onclick="setLocation('&lt;?php echo $this-&gt;getAddToCartUrl($_product) ?&gt;')"&gt;&lt;span&gt;&lt;span&gt;&lt;?php echo $this-&gt;__('Add to Cart') ?&gt;&lt;/span&gt;&lt;/span&gt;&lt;/button&gt;&lt;/span&gt;  
  &lt;?php else: ?&gt;
  </code></pre>
  
  <p>the above code i replaced by previous code(below code)</p>
  
  <pre><code>&lt;?php if($_product-&gt;isSaleable()): ?&gt;
  &lt;button type="button" class="button" onclick="setLocation('&lt;?php echo $this-&gt;getAddToCartUrl($_product) ?&gt;')"&gt;&lt;span&gt;&lt;span&gt;&lt;?php echo $this-&gt;__('Add to Cart') ?&gt;&lt;/span&gt;&lt;/span&gt;&lt;/button&gt;
  &lt;?php else: ?&gt;
  </code></pre>
  
  <p>now it is adding the quantity into the cart but i have to put manually into the box, i want to add + and - button for increasing and decreasing the quantity, what should i do.
  I tried lots of things but it is not working here. </p>