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/libslic3r/PrintObject.cpp

@@ -29,6 +29,7 @@
 #include "Fill/FillLightning.hpp"
 #include "Format/STL.hpp"
 #include "TreeSupport.hpp"
+#include "format.hpp"
 
 #include <float.h>
 #include <oneapi/tbb/blocked_range.h>
@@ -659,7 +660,7 @@ void PrintObject::generate_support_material()
                     {SharpTail,L("floating regions")},
                     {Cantilever,L("floating cantilever")},
                     {LargeOverhang,L("large overhangs")} };
-                std::string warning_message = format(L("It seems object %s has %s. Please re-orient the object or enable support generation."),
+                std::string warning_message = Slic3r::format(L("It seems object %s has %s. Please re-orient the object or enable support generation."),
                     this->model_object()->name, reasons[sntype]);
                 this->active_step_add_warning(PrintStateBase::WarningLevel::NON_CRITICAL, warning_message, PrintStateBase::SlicingNeedSupportOn);
             }