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/Utils/FileHelp.cpp

@@ -1,5 +1,6 @@
 #include "FileHelp.hpp"
 #include <boost/filesystem.hpp>
+#include <boost/log/trivial.hpp>
 #include <regex>
 namespace Slic3r {
     namespace Utils {
@@ -23,4 +24,4 @@ void slash_to_back_slash(std::string &file_path) {
     file_path = std::regex_replace(file_path, regex, "/");
 }
 
-}} // namespace Slic3r::Utils
+}} // namespace Slic3r::Utils