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/SelectMachine.cpp

@@ -20,6 +20,7 @@
 #include <wx/progdlg.h>
 #include <wx/clipbrd.h>
 #include <wx/dcgraph.h>
+#include <wx/mstream.h>
 #include <miniz.h>
 #include <algorithm>
 #include "Plater.hpp"