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/AxisCtrlButton.cpp

@@ -1,6 +1,8 @@
 #include "AxisCtrlButton.hpp"
 #include "Label.hpp"
+#include "libslic3r/libslic3r.h"
 
+#include <wx/dcclient.h>
 #include <wx/dcgraph.h>
 
 StateColor blank_bg(StateColor(std::make_pair(wxColour("#FFFFFF"), (int)StateColor::Normal)));
@@ -365,4 +367,4 @@ void AxisCtrlButton::sendButtonEvent()
     event.SetEventObject(this);
     event.SetInt(current_pos);
     GetEventHandler()->ProcessEvent(event);
-}
+}