model: implements MType::name

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
model: makes MParameter an MEntity

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
model: Generalize access to `model` form `MEntitiy`.

Since, MEntity are model things, let the user access to the model from them.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>