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

better scale management to avoid call getLegendGraphics everytime + some defaults + added code example to manage LegendURL when it will be correctly parsed - Developed with funding from Regione Toscana-SITA
added wms getLegendGraphic management in case of rasterLayerSymbology - Developed with funding from Regione Toscana-SITA
This commit add the getLegendGraphic pixmap as legend in the QgsLegend.
Due to architectural QgsLegend limitation pixmap is shown of the same dimension of every single legend element (too little)

managing of getLegendGraphic popup and refresh - Developed with funding from Regione Toscana-SITA
Due the fact that WMS pixmap is too little, has been added a popup to show WMS legend in the original size
Has been added feature to refresh getLegendGraphics to every scale change due to extend modification. In this way
getLagendGraphic can be updated basing on scale and in the future also basing on viewport.

Modified QPixmap to QImage to be device independent

set absolute position due to popup is not referred to legendInterface (avoid hiding of the legend)
Added a resolution parameter in option->Canvas&Legend to instruct WMS getLegendGraphic
to get WMS legend with a good resolution for printing.
Set value to 0 to get default pixmap resolution.

Drawback in case of big resolution of the legend, it becames too big to be
correctly shown as PopUp in the QgsLegend interface.

Developed with funding from Regione Toscana-SITA
Added managing of WMS legends (getLegendGraphic) in the addLegend of QgsComposer.
Added parameters to interactively modify wms legends scale.
Add WMS getLegendGraphic draw in composer and add it's size parameters in composer window.

Developed with funding from Regione Toscana-SITA
Merge pull request #966 from luipir/SITA3-WmsGetLegendGraphicManagement

WMS getLegendGraphic management in QgsLegend and QgsComposer - Developed with funding from Regione Toscana-SITA