Move Print object storage to C++. (along with its subobjects)
Add ::Ref classes for every missing C++ class (going by REGISTER_CLASS calls).
Return NULL values from XS as undef.
Try to fix _copies_shift scaling.
Remove some more Clone typemaps and add some whitespace.
Make read-only constructor parameters - private.
Set ISA manually for Slic3r::Layer::Support.
Make clear_regions and delete_region methods private (both Print & Layer), and remove from XSP.
Fix inherited Layer::Support id and object accessors.
Really fix total_used_filament / total_extruded_volume.
Add missing include "BoundingBox.hpp" to Polygon.xsp.
Attempt to solve conflict with a3bd1b5 by adding id to PrintObject and using that as a hash key.
Merge branch 'printcpp' of github.com:sapir/Slic3r into sapir-printcpp

Conflicts:
	lib/Slic3r/GCode.pm
	lib/Slic3r/Print.pm
	lib/Slic3r/Print/Object.pm
	lib/Slic3r/Print/Region.pm
Finish porting Print classes to XS
Update MANIFEST
Enable step caching
Use ->ptr() for CoolingBuffer too
Some initial work for incremental slicing
More work on incremental slicing
Make support material generation idempotent