Support for using base layer groups as layers in other Map Definitions

[zspitzer]

Allow base layer groups to be added like a layer in another mapdefinition.

This allows effective use of tile caches, because such a layer wouldn't refresh the tile cache on update of rest of the map definition

[jbirch]

Removing version from enhancement request.

I really like this idea BTW; base layer groups should be their own referencable resource type.

[jng]

Fixed with MapGuide RFC 140