Investigate how to handle const conrectness properly

[intjelic]

I open this ticket to keep track of this recurrent problem, wrapping constant instances...

For instance, we have sf.Texture that wraps around a sf::Texture instance. However, when the texture instance is constant, such as the one returned by sf::RenderTexture::getTexture(), we have to pay attention to:

• not destroying the underlying object when the wrapper is destroyed
• prevent access to non const methods

There's probably no clean solution to this issue but I'd like to write a document explaining the best practises to handle these situations.