Added getLegendGrafic feature to WmsDataProvider - Developed with funding from Regione Toscana-SITA

Added  getLegendGrafic feature to WmsDataProvider

getLegendGrafic has added to the RasterDataProvider base class too to allow visibility of the method. In case Raster is not a WMS it will return invalid QPixmap() (e.g QPixmap().isValid() == false)

This code add this feature and merge this other related commits 
- better scale management to avoid call getLegendGraphics everytime
- added some defaults
- added code example to manage LegendURL when it will be correctly parsed

Developed with funding from Regione Toscana-SITA