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/PrintConfig.cpp

@@ -20,6 +20,7 @@
 ///|/ PrusaSlicer is released under the terms of the AGPLv3 or higher
 ///|/
 #include "PrintConfig.hpp"
+#include "ClipperUtils.hpp"
 #include "Config.hpp"
 #include "I18N.hpp"