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/MarkdownTip.cpp

@@ -7,6 +7,8 @@
 #include "libslic3r/Utils.hpp"
 #include "I18N.hpp"
 
+#include <wx/display.h>
+
 namespace fs = boost::filesystem;
 
 namespace Slic3r { namespace GUI {