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.

tests/fff_print/test_trianglemesh.cpp

@@ -4,8 +4,6 @@
 #include "libslic3r/TriangleMesh.hpp"
 #include "libslic3r/TriangleMeshSlicer.hpp"
 #include "libslic3r/Point.hpp"
-#include "libslic3r/Config.hpp"
-#include "libslic3r/Model.hpp"
 #include "libslic3r/libslic3r.h"
 
 #include <algorithm>
@@ -17,9 +15,10 @@
 #include <vector>
 #include <math.h>
 #include "libslic3r/ExPolygon.hpp"
+#include "libslic3r/BoundingBox.hpp"
+#include "libslic3r/MultiMaterialSegmentation.hpp"
 
 //#include "test_options.hpp"
-#include "test_helpers.hpp"
 
 using namespace Slic3r;
 using namespace std;