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/Utils/json_diff.cpp

@@ -12,6 +12,10 @@
 
 #include <libslic3r/Utils.hpp>
 
+#include <boost/log/trivial.hpp>
+#include <boost/nowide/iostream.hpp>
+#include <boost/nowide/fstream.hpp>
+
 using namespace std;
 using json = nlohmann::json;