* unify the handling of models, tools, processes, tasks and (finally) toolpaths
* this required a change of the interfaces of
  pycam.Plugins.ObjectWithAttributes and pycam.Toolpath.Toolpath
  (use only named parameters - otherwise multiple inheritence fails)
* turn Toolpath.path into a property - now we can detect changes and clear the
  cache
* remove the Toolpath methods copy/crop/get_cropped_copy - now you can
  use the filter instead:
    toolpath | pycam.Toolpath.Filters.Crop(polygons)
* simplified the toolpath plugin - now the implementation is similar to
  models/tools/processes/tasks (no more information storage in gtk models)