Add attribute to QueryBuilder for feeding values combobox

In the QueryBuilder panel, one may use a combobox instead of a simple text field if the selected attribute has a "url"  property. It seems not possible to add this info directly in the mapfile/layer metadata.

This PR adds the info, using a new parameter in the plugin's config. For instance:
```
        attributeURLs: {
            "areal_liegenschaften": {
                "gemeinde": "${request.static_url('<project>:static/json/gemeinde.json')}"
            }    
        }
```
The project must expose some static files or webservices that give the value to add in the comboboxes. For instance:
```
{
    items: [
        {label: 'Liestal', value: 'Liestal'},
        {label: 'Sissach', value: 'Sissach'},
        {label: 'Lausen', value: 'Lausen'}
    ]
}
```