Replaced all wxString.ToStdString() with wxString.ToUTF8().data()

to be sure that the strings are correctly converted to UTF8.

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -126,7 +126,7 @@ public:
     }
 
     inline std::string get_name() const {
-        return fpath.GetName().ToStdString();
+        return fpath.GetName().ToUTF8().data();
     }
 
     template<class T> inline LayerWriter& operator<<(const T& arg) {