Remove Unused Project Includes and Forward-Declare Where a Type Is Only Referenced

Generated with include-what-you-use and applied conservatively. Only OrcaSlicer's own headers, the ones under src/ and tests/, are removed or forward-declared; standard-library and third-party includes are left alone. An include is removed only when both the Release and the Debug configuration leave it unused, never from inside a conditional block, and never from a file with platform-specific blocks, which only gain includes. Files whose only use of a header sits behind a feature or debug macro (libvgcode's OpenGL ES and marker code, the ARACHNE/TESTS_EXPORT_SVGS debug output) keep their includes.

clonable_ptr.hpp gains #pragma once; it had no include guard and was only safe while Config.hpp was its sole includer.

src/libslic3r/GCode.hpp

@@ -4,10 +4,8 @@
 #include "ExtrusionEntity.hpp"
 #include "Polygon.hpp"
 #include "Config.hpp"
-#include "ExtrusionEntityCollection.hpp"
 #include "Print.hpp"
 #include "libslic3r.h"
-#include "ExPolygon.hpp"
 #include "GCodeWriter.hpp"
 #include "Layer.hpp"
 #include "Point.hpp"
@@ -22,7 +20,6 @@
 #include "GCode/WipeTower.hpp"
 #include "GCode/SeamPlacer.hpp"
 #include "GCode/GCodeProcessor.hpp"
-#include "EdgeGrid.hpp"
 #include "GCode/ThumbnailData.hpp"
 #include "libslic3r/ObjectID.hpp"
 #include "GCode/ExtrusionProcessor.hpp"
@@ -48,6 +45,10 @@
 #include <cfloat>
 #include <vector>
 #include <utility>
+#include "BoundingBox.hpp"
+#include "Polyline.hpp"
+
+namespace Slic3r { class ExtrusionEntityCollection; }
 
 namespace Slic3r {