radio in tree uses image from default theme

So if you're using another theme such as Neptune, it won't look nice.

For now I'm using a css override but interested to hear ideas of something that could work for any theme.

```
.gx-tree-radio {
    background-image: url('http://docs.sencha.com/extjs/4.2.2/extjs-build/resources/ext-theme-neptune/images/form/radio.png');
}
```