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/PrivacyUpdateDialog.hpp

@@ -6,6 +6,7 @@
 #include "Widgets/Label.hpp"
 #include "Widgets/WebView.hpp"
 #include <wx/webview.h>
+#include <wx/progdlg.h>
 #include <wx/simplebook.h>
 
 namespace Slic3r { namespace GUI {
@@ -53,4 +54,4 @@ public:
 
 }} // namespace Slic3r::GUI
 
-#endif
+#endif