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/MainFrame.hpp

@@ -26,6 +26,8 @@
 #include "Widgets/SideButton.hpp"
 #include "Widgets/SideMenuPopup.hpp"
 
+#include <boost/property_tree/ptree_fwd.hpp>
+
 // BBS
 #include "BBLTopbar.hpp"
 #include "PrinterWebView.hpp"