Squash merge of lh_brim_rework,

brim separated to Brim.cpp,hpp
Refactored accessors to PrintObjectPtrs, PrintRegionPtrs, LayerPtrs,
SupportLayerPtrs for const correctness.

src/libslic3r/GCode.hpp

@@ -33,7 +33,7 @@ class GCode;
 
 namespace { struct Item; }
 struct PrintInstance;
-using PrintObjectPtrs = std::vector<PrintObject*>;
+class ConstPrintObjectPtrsAdaptor;
 
 class OozePrevention {
 public: