[QGIS-Server] FIX 10489 QGIS Server getcapabilites response is not WMS compliant

To be fully compliant with WMS 1.3.0 standard, I propose to add :
* SLD namespace for GetLegendGraphic element
* schemaExtension.xsd distributed with QGIS-Server for GetStyles and GetPrint
* GetSchemaExtension to retrieve schemaExtension.xsd like MapServer

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