Added WMS getLegendGraphic to WMS Provider - Developed with funding from Regione Toscana-SITA

This request adds getLegendGrahic API to WMS Provider and it's managing.

getLegendGrahic manage silently if WMS server can or can not support getLegendGraphic.
getLegendGrahic call return a valid or not QPixmap() based on scale parameter.

To allow access to this api through the generic RasterDataProvider it's signiture has been added.

This feature has been  developed with funding from Regione Toscana-SITA