Deleted unsafe default copy constructors / operators in Model / ModelObject / ModelInstance / ModelVolume
Fixed some issues with copying the Model / ModelObject / ModelInstance / ModelVolume inside Print::apply()
Fixed some invalidation issues in Print::apply()
Temporarily renamed the Slic3rPE profile directory to Slic3rPE-alpha.
added unique IDs to the Model / ModelObject / ModelVolume objects,
added a copy of Model hierarchy at the Print,
WIP: new Print::apply() method to update the Print's copy of Model,
to update the Print's PrintObjects, to update status of the Print and
PrintObjects, and to possibly stop the background processing.