Enabled the export of information of WFSGrid in CSV format

Enabled the export of information of WFSGrid in CSV format

Is possible to enable or disable the export in config

.....
{
	"ptype": "gxp_featuregrid",
	"featureManager": "featuremanager",
	"outputConfig": {
		"id": "featuregrid",
		"title": "Features"
	},
	"outputTarget": "south",
        "showExportCSV": true    <-----
}
.....