Added support for LayerGroup to the WMS GetLegendGraphic action

LayerGroups are supported in the following manner:
 - just use a layergroup name, instead of a layer name, in the LAYER parameter to get a legend for the group
 - if STYLE is not specified, the default style is used for all the layers of the group
 - if STYLE is specified it could be:
   * a single value, used for all the group layers
   * a comma separated list to specify styles for any single layer of the group (if less styles than layers are specified the last style will be used for the remaining layers)
   * a comma separated list with empty values to use specific styles for some layers and default for the ones left empty (for example you can specify style1,,style3 to use the default style for the second layer)
 - the RULE parameter is used similarly to STYLE to specify rules for some or all the styles
 - the layer Title (or Name if title is missing) is drawn as a label on top of each layer