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/TempInput.hpp

@@ -3,6 +3,7 @@
 
 #include "../wxExtensions.hpp"
 #include <wx/textctrl.h>
+#include <wx/stattext.h>
 #include "StaticBox.hpp"
 
 wxDECLARE_EVENT(wxCUSTOMEVT_SET_TEMP_FINISH, wxCommandEvent);