value handling needs to be fixed

The file LinucCNC.py is supposed to be a good example for a machine-specific
language export definition.

* this allows to handle the filters separately instead of applying the immediately

* filters in Toolpath are now read-only and protected by property functions
* change of filters -> clear toolpath cache
* add "comment" as new toolpath record

* added "weight" attribute to allow sorting
* added PathMode (G61/G64) filter
* fixed typos in TriggerSpindle filter
* fixed typos in StepWidth filter

* SelectTool
* TriggerSpindle
* StepWidth

* this currently fails to expose the tool radius to the path processor

* many events are not grepped due to linebreaks

* fixed source directory
* moved the script to "scripts/"
* ignore long lines

Otherwise the "last" (based on the order of plugin initialization) toolpath
processor is selected. The load order is arbitrary - thus the initial processor
selection not determined. (we don't like this)

* tabs in the gtk.Notebook are now hidden, if all children are disabled
* removed previous (ugly) attempt of hiding empty parents
* added path_mode and similar attributes to the "Laser" postprocessor
* improved layout of "settings" tab in gtk.Notebook

* additionally: hide a ParameterSection if all children are hidden

* replaced by toolpath_processors.ui

* ToolpathProcessors now handles different kinds of tasks (Milling, Cutting, ...)
* all settings in GCodeParameters are moved to the proper ParameterGroup
* for now we ignore the PostProcessors (different GCode dialects)
* show/hide does not work currently for "spindle_enable"
* settings are currently not applied - just the UI works
* still no GCode export

* proper teardown
* selective show for input controls
* use ".get_widget()" instead of the internal ".widget" property

Use a separate function (get_non_conflicting_name) for determining the
name of new objects (models, processes, bounds, tasks and toolpaths).

* TaskTypeMilling returns a tuple of (moves, parameters) - instead of a
  complete Toolpath instance
* ToolpathCrop uses the Crop filter instead of the object methods for cropping
* ToolpathGrid uses the Copy filter instead of just calling "list(..)"
* ToolpathCrop and ToolpathGrid use "add_new" instead of "append" for new
  Toolpaths

* 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)

* MovesOnly: useful for determining if a toolpath is empty/useless
* Copy: just hide the interal structure of a toolpath (tuple/list)

* duplicate code removed
* marked as gtk-specific

* skip move to safety height if we go straight down afterwards