WMS getLegendGrapic popup in LegendInterface - Developed with funding from Regione Toscana-SITA

This pull request add the getLegendGraphic pixmap as legend in the QgsLegend and it's visualization using a PopUp gui.

Due to architectural QgsLegend limitation, pixmap is shown of the same dimension of every single legend element (too little). Due to this fact, has been added a popup to show WMS legend in the original size.
Has been added  a listener slot to refresh legends (and 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.

All this has bee developed with funding from Regione Toscana-SITA