added LegendURL to Layers style section

this commit adds a <LegendURL> to each layer in STYLE to make QGIS mapserver work with clients reading GetLegend URLs directly from each LAYER. E.g. http://geoportal.bayern.de/bayernatlas
This format can also be seen on p. 42 of OGC 05-078r4 (http://www.opengeospatial.org/standards/sld SLD Version 1.1.0)

A great additional feature could be to make the URL alternatively configurable in QGIS project settings to link to a custom made URL (graphic).