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/libnest2d/test_nfp.cpp

@@ -13,6 +13,8 @@
 #include <catch2/matchers/catch_matchers.hpp>
 #include <catch2/matchers/catch_matchers_floating_point.hpp>
 #include "libnest2d_test_utils.hpp"
+#include "libslic3r/ExPolygon.hpp"
+#include "libslic3r/Point.hpp"
 
 using namespace libnest2d;