Add a config parameter for the styles of hilighted features in FeaturesGrid

As for now, it is not possible to customize the way the features listed in the FeaturesGrid are displayed on the map.

This PR suggests to add a ``highlightStyle`` plugin config parameter, the same way it is done with [FeaturesWindow](https://github.com/camptocamp/cgxp/blob/master/core/src/script/CGXP/plugins/FeaturesWindow.js#L200-220).

Example of configuration:
```
{
    ptype: "cgxp_featuresgrid",
    highlightStyle: {
        "fillColor": "red"
    }
}
```