[Experimental]: start using JAXB to store configurations
moved current configuration to its own class

* disallow access to ExtruderSettings through array
bugfix: NPE when accessing attributes without material
get the extruder settings from CurrentConfiguration ...

... instead of asking the printer for extruder by material

* Mixed also a bugfix with this commit: ExtrusionOverrun is a double
  and should keep that name.
* Removed write-only firstExtruder and lastExtruder in LayerRules
minor cleaning
refactored slice() method, use a Map and ...

... an inner class to keep csgs, edges and attribute together 
instead of three arrays indexed by the extruder id.
LayerProducer takes a PolygonList to plot

no PolygonList[] at construction time.
print the foundation layer with the support extruder