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_helpers.cpp

@@ -5,8 +5,6 @@
 #include "libslic3r/GCodeReader.hpp"
 #include "libslic3r/Config.hpp"
 #include "libslic3r/Print.hpp"
-#include "libslic3r/Format/OBJ.hpp"
-#include "libslic3r/Format/STL.hpp"
 
 #include <algorithm>
 #include <cstdlib>
@@ -488,6 +486,14 @@ std::vector<std::string> role_sequence(const std::string &gcode, const std::vect
 } } // namespace Slic3r::Test
 
 #include <catch2/catch_all.hpp>
+#include "libslic3r/Arrange.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/Arrange.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/Arrange.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/Arrange.hpp"
+#include "libslic3r/Model.hpp"
 
 SCENARIO("init_print functionality", "[test_helpers]") {
 	GIVEN("A default config") {