Added support for LayerGroup to the WMS GetLegendGraphic action

Improvement GEOS-5524

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 should be a comma separated list to specify styles for any single layer of the group (empty values can be given to use default style for one or more layers)
 - 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
 - forceTitles legend option can be used to force layer titles off or on