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/GUI_App.cpp

@@ -28,6 +28,8 @@
 #include <boost/lexical_cast.hpp>
 #include <boost/log/trivial.hpp>
 #include <boost/nowide/convert.hpp>
+#include <boost/property_tree/ptree.hpp>
+#include <boost/property_tree/json_parser.hpp>
 
 #include <wx/stdpaths.h>
 #include <wx/imagpng.h>