[BUGFIX] WMS GetCapabilities interoperability

To be fully compliant with WMS 1.3.0 standard, I add :
* SLD namespace for GetLegendGraphic element
* GetSchemaExtension to retrieve schemaExtension.xsd like MapServer for GetStyles

The solution was inspired by
 http://demo.mapserver.org/cgi-bin/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetCap$
And the standard description
Styled Layer Descriptor profile of the Web Map Service Implementation Specifica$
http://portal.opengeospatial.org/files/?artifact_id=22364

A pull request has been proposed for QGIS-Server but this bug fix is also for
 previous QGIS-Server version.