Config: Make Load() and Save() slightly faster

Only invoke config changed callbacks from Config::Save, not
Layer::Save. The latter results in callbacks being called
once per layer, up to 7 times per save.

Source/Core/Common/Config/Config.cpp

@@ -62,12 +62,14 @@ void Load()
 {
   for (auto& layer : s_layers)
     layer.second->Load();
+  InvokeConfigChangedCallbacks();
 }
 
 void Save()
 {
   for (auto& layer : s_layers)
     layer.second->Save();
+  InvokeConfigChangedCallbacks();
 }
 
 void Init()