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/Printer/PrinterFileSystem.cpp

@@ -7,10 +7,14 @@
 #include "../../Utils/NetworkAgent.hpp"
 #include "../BitmapCache.hpp"
 
+#include <boost/endian/conversion.hpp>
+#include <boost/log/trivial.hpp>
 #include <boost/algorithm/hex.hpp>
 #include <boost/uuid/detail/md5.hpp>
 #include <boost/regex.hpp>
 
+#include <wx/mstream.h>
+
 #include "nlohmann/json.hpp"
 
 #include <cstring>