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/Http.hpp

@@ -1,11 +1,15 @@
 #ifndef __Http_hpp__
 #define __Http_hpp__
 
+#include <map>
 #include <memory>
 #include <string>
 #include <functional>
 #include <boost/filesystem/path.hpp>
 
+#include "libslic3r/Exception.hpp"
+#include "libslic3r_version.h"
+
 #define MAX_SIZE_TO_FILE    3*1024
 
 namespace Slic3r {