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/Widgets/StateHandler.cpp

@@ -1,4 +1,5 @@
 #include "StateHandler.hpp"
+#include <wx/window.h>
 
 wxDEFINE_EVENT(EVT_ENABLE_CHANGED, wxCommandEvent);