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/SliceInfoPanel.cpp

@@ -1,5 +1,6 @@
 #include "SliceInfoPanel.hpp"
 
+#include <boost/log/trivial.hpp>
 #include "I18N.hpp"
 #include "Widgets/Label.hpp"
 #include "libslic3r/Utils.hpp"
@@ -344,4 +345,4 @@ void SliceInfoPanel::msw_rescale()
 
 
 }
-}
+}