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/BBLTopbar.cpp

@@ -1,6 +1,7 @@
 #include "BBLTopbar.hpp"
 #include "wx/artprov.h"
 #include "wx/aui/framemanager.h"
+#include "wx/display.h"
 #include "I18N.hpp"
 #include "GUI_App.hpp"
 #include "GUI.hpp"
@@ -10,6 +11,8 @@
 #include "WebViewDialog.hpp"
 #include "PartPlate.hpp"
 
+#include <boost/log/trivial.hpp>
+
 #define TOPBAR_ICON_SIZE  18
 #define TOPBAR_TITLE_WIDTH  300