Parameter for max number of attributes cols while printing

Attributes tables printed in the PDF/PNG output are always limited to 9 columns, even if additional columns are added to the "columnDefs" in ``print/config.yaml`` (by the way the standard print config has a col range going from "col0" to "col9", which makes 10 columns).

The reason of this behaviour is that plugins FeaturesGrid and FeaturesWindow have an hard-coded limitation to 9 cols:
```
                    for (var prop in attributes) {
                        if (attributes.hasOwnProperty(prop)) {

                            var id = 'col' + index;
                            raw[id] = attributes[prop];
                            index++;
                            if (index > 9) {
                                break;
                            }
                            if (groupedRecords[grid.title]._newGroup) {
                                groupedRecords[grid.title][id] = OpenLayers.i18n(prop);
                                groupedRecords[grid.title].table.columns.push(id);
                            }
                        }
                    }
```
See https://github.com/camptocamp/cgxp/blob/master/core/src/script/CGXP/plugins/FeaturesGrid.js#L364

The present PR suggests to replace this hard-coded value by a ``maxPrintedColumns`` parameter that defaults to 9 and can be set in the plugin's config. For instance:
```
    { 
        ptype: "cgxp_featuresgrid",
        id: "featureGridBL",
        featureManager: "featuremanager",
        csvURL: "${request.route_url('csvecho')}",
        maxFeatures: 200, 
        themes: THEMES,
        outputTarget: "featuregrid-container",
        maxPrintedColumns: 11,
        events: EVENTS
    }
```

On the other hand I have tried to completely remove the ``index > 9`` test and it seems to work fine with layers coming with whatever number of attributes. The final number of printed attributes is then only configured in ``print/config.yaml`` which seems more relevent.

Is there a good reason to keep this test in FeaturesGrid and FeaturesWindow?