Add entities management

[SuperFola]

We should be able to

• render / update them (thus they should be part of the scene graph)
• send events to them

Maybe we will need a a specific event loop related to the game (for the events we can send to the entities)

[SuperFola]

could be based on https://github.com/SuperFola/Pataro actors (entities), sending actions to be executed.

but for performance reasons it would be quite slow