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/GLTexture.cpp

@@ -33,6 +33,7 @@
 
 #include "libslic3r/Utils.hpp"
 #include "GUI_App.hpp"
+#include <boost/log/trivial.hpp>
 #include <wx/dcgraph.h>
 namespace Slic3r {
 namespace GUI {