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/DropDown.cpp

@@ -1,6 +1,8 @@
 #include "DropDown.hpp"
 #include "Label.hpp"
 
+#include <wx/display.h>
+#include <wx/dcbuffer.h>
 #include <wx/dcgraph.h>
 
 #ifdef __WXGTK__