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_plugin_host_api.cpp

@@ -7,7 +7,6 @@
 #include <slic3r/GUI/DockPanel.hpp>
 #include <slic3r/GUI/AuiPaneLayout.hpp>
 #include <slic3r/GUI/Widgets/WebHosting.hpp>
-#include <slic3r/plugin/PythonPluginBridge.hpp>
 
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/catch_message.hpp>
@@ -25,6 +24,11 @@
 
 #include <wx/string.h>
 #include <wx/uri.h>
+#include "libslic3r/ObjectID.hpp"
+#include "libslic3r/Point.hpp"
+#include "libslic3r/Preset.hpp"
+#include "libslic3r/PrintConfig.hpp"
+#include <pybind11/detail/common.h>
 
 namespace py = pybind11;