cannot specify listeners in outputConfig of layertree plugin

since this will override the listeners that the code in the layertree plugin is creating:


```
            listeners: {
                contextmenu: this.handleTreeContextMenu,
                beforemovenode: this.handleBeforeMoveNode,
                scope: this
            },
```

```

                ptype: "gxp_layertree",
                outputConfig: {
                    id: "layertree",
                    enableDD:true,
                    listeners: {
                        'nodedrop': function(evt) {
                            ..
                        },
                        scope: this
                    }
                },
                outputTarget: "tree"
```