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.cpp

@@ -132,6 +132,14 @@ using namespace std::literals::string_view_literals;
 #endif
 
 #include <assert.h>
+#include "AABBTreeLines.hpp"
+#include "Extruder.hpp"
+#include "ExtrusionEntityCollection.hpp"
+#include "FilamentMixer.hpp"
+#include "Format/STEP.hpp"
+#include "Model.hpp"
+#include "MultiNozzleUtils.hpp"
+#include "Slicing.hpp"
 
 namespace fs = boost::filesystem;