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/slic3rutils/test_printer_agent.cpp

@@ -1,10 +1,7 @@
 #include <catch2/catch_all.hpp>
 
 #include <memory>
-#include "slic3r/Utils/ICloudServiceAgent.hpp"
-#include "slic3r/Utils/IPrinterAgent.hpp"
 #include <slic3r/Utils/NetworkAgentFactory.hpp>
-#include <slic3r/plugin/PythonPluginBridge.hpp>
 
 #include <catch2/catch_test_macros.hpp>
 #include <pybind11/pytypes.h>
@@ -15,6 +12,10 @@
 #include <pybind11/pybind11.h>
 
 #include <string>
+#include <pybind11/cast.h>
+
+namespace Slic3r { class ICloudServiceAgent; }
+namespace Slic3r { class IPrinterAgent; }
 
 using namespace Slic3r;
 namespace py = pybind11;