Squash merge of lh_brim_rework,

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

src/libslic3r/Brim.hpp

@@ -0,0 +1,14 @@
+#ifndef slic3r_Brim_hpp_
+#define slic3r_Brim_hpp_
+
+namespace Slic3r {
+
+class Print;
+
+// Produce brim lines around those objects, that have the brim enabled.
+// Collect islands_area to be merged into the final 1st layer convex hull.
+ExtrusionEntityCollection make_brim(const Print &print, PrintTryCancel try_cancel, Polygons &islands_area);
+
+} // Slic3r
+
+#endif // slic3r_Brim_hpp_