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/libslic3r/Shape/TextShape.cpp

@@ -28,6 +28,8 @@
 #include "BRepPrimAPI_MakePrism.hxx"
 #include "Font_FontMgr.hxx"
 
+#include <boost/log/trivial.hpp>
+
 namespace Slic3r {
 
 static std::map<std::string, std::string> g_occt_fonts_maps; //map<font_name, font_path>