Use ogcproxy to get KML icons

This PR replaces `<href> http......</href>`, in KML file, by `<href>{ogcproxyUrl}http...`.
This allow non-CORS image to be used.
 

Must be merged after https://github.com/openlayers/ol3/issues/1603 and https://github.com/geoadmin/mf-chsdi3/pull/373