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/AvoidCrossingPerimeters.cpp

@@ -31,6 +31,11 @@
 #include <boost/range/adaptor/reversed.hpp>
 #include <vector>
 #include <utility>
+#include "libslic3r/GCode/AvoidCrossingPerimeters.hpp"
+#include "libslic3r/Extruder.hpp"
+#include "libslic3r/GCodeWriter.hpp"
+#include "libslic3r/PrintConfig.hpp"
+#include "libslic3r/SurfaceCollection.hpp"
 
 namespace Slic3r {