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.cpp

@@ -1,4 +1,5 @@
 #include "ErrorMsgStaticText.hpp"
+#include <wx/dcclient.h>
 
 ErrorMsgStaticText::ErrorMsgStaticText() {}
 
@@ -51,4 +52,4 @@ void ErrorMsgStaticText::paintEvent(wxPaintEvent &evt)
     SetMinSize(wxSize(-1, line_count * text_height));
     SetMaxSize(wxSize(-1, line_count * text_height));
     dc.DrawText(out_txt, 0, 0);
-}
+}