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/libslic3r/GCodeReader.cpp

@@ -1,6 +1,7 @@
 #include "GCodeReader.hpp"
 #include <boost/algorithm/string/classification.hpp>
 #include <boost/algorithm/string/split.hpp>
+#include <boost/log/trivial.hpp>
 #include <boost/nowide/fstream.hpp>
 #include <boost/nowide/cstdio.hpp>
 #include <fstream>