Initial work for new model handling
Merge branch 'master' into xs-model
Initial work for porting Slic3r::Config to XS
More work for refactoring add_model() to add_model_object(). Tests pass
Refactoring. Use Model class for representing the plate in GUI
Make quick_slice() work with new model handling
Merge branch 'xs-model'

Conflicts:
	lib/Slic3r/Model.pm
Use 0,0 for default instance
Some initial work for stateful Print object
Move centering outside the Print object
Some initial work for adapting plater to the new centering workflow
Finished updating plater to the new centering logic
More work for updating the embedded Print object when Model is changed in plater
More refactoring to clean up the Print object API
Removed extra line
Handle read_from_file() exceptions in GUI. #1619
Handle read_from_file() exceptions in GUI. #1619
Fixed fatal error about missing method thin_walls(). #1607 #1615
Lazy mesh generation so that we only keep it in model object
Fixed wrong commit f495136. Thanks pdbogen for the report. #1623