ZoomBox config options.

Now you can add only one action for this plugin.

You must remove the default one with `"removeTools":["zoombox_plugin"]` and append the customized one:

```
    {
        "ptype": "gxp_zoombox",
        "id": "custom_zoombox_plugin",
        "toggleGroup": "toolGroup",
        "appendZoomOut": false,
        "actionTarget": {
        	"target": "paneltbar",
        	"index": 4
        }
    }
```

The options are:

* **appendZoomIn**: Flag to append zoomIn. Default it's true
* **appendZoomOut**: Flag to append zoomOut. Default it's true
Merge pull request #344 from alediator/zoombox_options

ZoomBox config options.