Handle opacity setting for MapLayers

Added a superclass for all MapLayer Parameter objects which has an opacity.

Based on that opacity update the Geotools style (before rendering) so that the opacity is applied to each stroke, fill and graphic.  The way the opacity is applied is to multiply it (a value between 0 and 1)  by the value in the style to get the adjusted value.  IE if a style has an opacity of `.7` and the opacity of the layer is `0.2` then the combined opacity is `0.7 * 0.2 = 0.14`

A short cut is also added that checks if the opacity is 0.  If it is then the layer is completely skipped.