Add the ability to 'unpack' and serve a layer group's layers as individual <map-extent> elements

[prushforth]

This issue creates a new text/mapml format option parameter named mapmlmultiextent, that reflects the (currently global) WMS setting to serialize text/mapml documents containing multiple <map-extent> elements.

A layer group may contain other layer groups, or style groups, which in turn contain other layer groups, and so on. These layers and groups are all eventually rendered on the same canvas, normally a bitmap graphics buffer. When a GetMap is received containing the format_options=mapmlmultiextent:false parameter (the default), GeoServer renders the result as a single <map-extent> configured according to the mapmlusetiles:true|false and mapmlusefeatures:true|false format_option value. If the format_options=mapml:multiextent:true parameter is received, when processing a LayerGroup, GeoServer should render each layer in the group as an individual <map-extent> in the response document, each of which is also configured in respect of the received mapmlusetiles:true|false and mapmlusefeatures:true|false format_option values.