GetLegendGraphics with no label

With this commit, you can call GetLegendGraphics with two parameters (LAYERTITLE=false / RULELABEL=false) to disable labels in the resulting legend.
Useful for Qgis Web Client, LizMap-Web-Client and Qgis2Leaf.
GetLegendGraphics with no label

Added two private variables mDrawLegendLayerLabel and mDrawLegendItemLabel
Merge pull request #1377 from luca76/master

WMS GetLegendGrapics: obtain legend graphics with no labels