add icon class to tree-node based on context, id, and parent

add icon class to tree-node based on context, id, and parent

case: https://github.com/goldsky/GridClassKey/issues/72
![tree-node-icons](https://cloud.githubusercontent.com/assets/308524/2739055/4e850e54-c6a0-11e3-97b7-eac597044f8c.png)

```css
div.icon-web-82 .x-tree-node-icon {
    background-image: url(../icons/shop.png) !important;
    background-repeat: no-repeat;
}

div.icon-web-83 .x-tree-node-icon {
    background-image: url(../icons/cart.png) !important;
    background-repeat: no-repeat;
}

div.icon-web-84 .x-tree-node-icon {
    background-image: url(../icons/box.png) !important;
    background-repeat: no-repeat;
}

div.icon-web-85 .x-tree-node-icon {
    background-image: url(../icons/factory.png) !important;
    background-repeat: no-repeat;
}
```

Btw,
I'm bringing this pull request to master, because the develop branch has this file way a lot different and I don't want to break any new changes.