Day 101: Scenes manager improvements

Today I did a few things to improve the user-friendlyness of the scenes manager.

First I created a file in the game folder where you can associate a name to each class:

SceneManager::add_scene("myscene");

If you want to use a scene in the editor, it must be declared there, more or less like in Unity.
This allows to change scene by name, which is way better than what I was doing until now.
In the meanwhile I also fixed a bad bug that made the game crash if no object is rendered, which never occurred until now.

Finally I added e \setscene command to the internal console (picture), so I can easily switch scene instead of cycling to all of them.