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/ErrorMsgStaticText.hpp

@@ -2,6 +2,7 @@
 #ifndef _WX_ERRORMSGSTATTEXT_H_
 #define _WX_ERRORMSGSTATTEXT_H_
 
+#include <wx/panel.h>
 #include "wx/stattext.h"
 
 class WXDLLIMPEXP_CORE ErrorMsgStaticText : public wxPanel
@@ -19,4 +20,4 @@ public:
     void SetLabel(wxString msg){m_msg = msg;};
 
 };
-#endif
+#endif