Feature grid improvements

* Add `exportFormats` and `exportFormatsConfig` options to this widget.
* Add images and styles for the new exports
* Minor fix on Circle spatial selector.

You can test it adding the feature grid with this configuration:

```
	    {
		  "ptype": "gxp_featuregrid",
		  "featureManager": "featuremanager",
		  "outputConfig": {
			  "id": "featuregrid",
			  "title": "Features"
		  },
		  "outputTarget": "south",
		  "exportFormats": ["CSV","shape-zip","excel", "excel2007"]
	    }
```