Improve KML parsing and display of GeometryCollection

This PR makes 2 things:

  - Add the possibility to parse a `<LinearRing>`  outside a `<Polygon>`

  - Fix the display of a GeometryCollection with all types of geometries available

I've updated the kml.html example to test these features.


I will be happy to have a review, in particular on all the modifications I've done in XXXLiteral  and XXXSymbolizer classes.