Remove Duplicate Includes and the Dead GCodeSender Sources (#16249)

46 files include the same header twice at file scope, outside any #if,
66 times in all:
Model.cpp included Model.hpp twice, Utils.hpp <algorithm> and
<string_view> twice, seven GUI headers <wx/dataview.h> and
<wx/artprov.h> twice. The second include of each is removed.

GCodeSender.cpp and GCodeSender.hpp have been commented out of
libslic3r/CMakeLists.txt since 2022 and their only two includes are
commented out as well. Both files go, with the commented lines, and
the CMake entry for SLA/SupportTreeIGL.cpp, a file that no longer
exists.

src/slic3r/Utils/EmbossStyleManager.cpp

@@ -365,7 +365,6 @@ void StyleManager::init_trunc_names(float max_width) {
 #include "slic3r/GUI/Plater.hpp" 
 
 // for get DPI
-#include "slic3r/GUI/GUI_App.hpp"
 #include "slic3r/GUI/MainFrame.hpp"
 #include "slic3r/GUI/Gizmos/GizmoObjectManipulation.hpp"
 
@@ -554,8 +553,6 @@ bool StyleManager::set_wx_font(const wxFont &wx_font, std::unique_ptr<FontFile>
     return true;
 }
 
-#include <libslic3r/AppConfig.hpp>
-#include "WxFontUtils.hpp"
 #include "fast_float/fast_float.h"
 
 // StylesSerializable