Update modx.grid.js

When you enable grouping on a MODx.grid.Grid, it will apply a Ext.grid.GroupingView to the grid, which is great feature, but as I want to hide/disable the "group by this column" feature when you click on the little arrow near each column header, I need to pass "enableGroupingMenu: false" to the GroupingView. But I cannot do that (whithout touching the core files).

Maybe good to have a "groupingConfig: { }" which will be passed when you set this up in your grid. This is how it works for me.
```
applyIf( obj, config ) : Object
Copies all the properties of config to obj if they don't already exist.
```
so now it takes all props from config.groupingConfig and insert them to var groupingConfig, but only if they don't exists there (which means you can't overwrite the defaults that you set when you initialised var groupingConfig).

My suggestion was:
```javascript
if (config.grouping) {
        var groupingConfig = {
            forceFit: true
            ,scrollOffset: 0
            ,groupTextTpl: '{text} ({[values.rs.length]} {[values.rs.length > 1 ? "'
                +(config.pluralText || _('records')) + '" : "'
                +(config.singleText || _('record'))+'"]})'
            };

        Ext.applyIf(config.groupingConfig, groupingConfig);

        Ext.applyIf(config,{
          view: new Ext.grid.GroupingView(config.groupingConfig)
        });
    }
```

which allows you to overwrite even the defaults (such as groupTextTpl).