Fix of the "dock" menu on OSX to start a new instance of PrusaSlicer

in case a single instance is disabled.
Make the "single_instance" enabled by default on OSX initially
to match the platform.

src/libslic3r/AppConfig.cpp

@@ -74,7 +74,13 @@ void AppConfig::set_defaults()
 #endif
 
         if (get("single_instance").empty())
-            set("single_instance", "0");
+            set("single_instance", 
+#ifdef __APPLE__
+                "1"
+#else __APPLE__
+                "0"
+#endif __APPLE__
+                );
 
         if (get("remember_output_path").empty())
             set("remember_output_path", "1");