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/AuxiliaryDialog.cpp

@@ -4,6 +4,8 @@
 
 #include "libslic3r/Utils.hpp"
 
+#include <boost/property_tree/ptree.hpp>
+
 namespace pt = boost::property_tree;
 typedef pt::ptree JSON;
 
@@ -33,4 +35,4 @@ void AuxiliaryDialog::on_dpi_changed(const wxRect& suggested_rect)
 }
 
 } // namespace GUI
-} // namespace Slic3r
+} // namespace Slic3r