Bug 1037655 - Move popup icon live preview into a preview icon in the editor 

Yeah, we need to set this to whatever is in _popcornOptions.icon, or hide it if it's set to "none"

I think we need an updateIcon function, that looks something like this:

```
        function updateIcon(iconImg, value) {
          if ( value === "none" ) {
            iconImg.style.visibility = "hidden";
            iconImg.src = "";
          }
          else {
            iconImg.src = "/src/plugins/popup/images/" + value + ".png";
            iconImg.style.visibility = "initial";
          }
        }
```

And update the iconHandler function to call it:

```
        function iconHandler() {
          var that = this;
          setTimeout(function() {
            updateIcon( that.parentNode.querySelector( ".popup-icon-preview" ), that.value);
          });
        }
```

Then we can call updateIcon when we setup the keyup and mouseover events, like this:

```
updateIcon( iconImg, _popcornOptions.icon );
```