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/WebViewDialog.cpp

@@ -7,6 +7,9 @@
 #include "libslic3r_version.h"
 #include "../Utils/Http.hpp"
 
+#include <boost/property_tree/ptree.hpp>
+#include <boost/property_tree/json_parser.hpp>
+
 #include <wx/sizer.h>
 #include <wx/toolbar.h>
 #include <wx/textdlg.h>