Implement a framework for foliage

Create a framework which will be used for generating natural looking foliage and undervegetation. The vegetation should be rendered with the PagedGeometry engine.
All settings should be defined in a datafile, presumably .terrain.
The vegetation should use the layers for determining placement and type, so that no grass grows on paths and so on.
The long term goal is to use Mercator for all foliage placement, but in the mean time we can provide our own placement techniques. This can also be used as a testbed for different techniques which later on can be implemented in Mercator.