A *lot* of import fixes for flatpak (#3909)

* A *lot* of import fixes for flatpak

Aside, thank you @hadess for the majority of these fixes. You are the
base point for a lot of issues fixed during the creation of this
flatpak.

* Use slic3r::load_string_file

Boost 1.84 removed `boost::fs::load_string_file` so use the func located
in Utils.hpp

src/slic3r/GUI/WipeTowerDialog.cpp

@@ -13,8 +13,6 @@
 #include "MainFrame.hpp"
 #include "libslic3r/Config.hpp"
 
-#include <wx/sizer.h>
-
 using namespace Slic3r;
 using namespace Slic3r::GUI;