WMS getLegendGrapic in Composer - Developed with funding from Regione Toscana-SITA

This commit add different elements to allow correct visualizing of a WMS getLegendGrapic in the QgsComposer. This elements are:

1) A resolution parameter in option->network to instruct WMS getLegendGraphic to get WMS legend with a good resolution for printing
2) Composer parametes to interactively modify wms legends size
3) code to show getLegendGrahic as legend layer. It's is added if it's available in QgsLegend interface.

This feature has been developed with funding from Regione Toscana-SITA