ENH: save uniform scale to an app_config item

Change-Id: Ib4a42597308382ea195d73776790e9cd4a122b76

src/libslic3r/AppConfig.cpp

@@ -299,6 +299,10 @@ void AppConfig::set_defaults()
     }
 #endif
 
+    if (get("uniform_scale").empty()) {
+        set("uniform_scale", "1");
+    }
+
     // Remove legacy window positions/sizes
     erase("app", "main_frame_maximized");
     erase("app", "main_frame_pos");