model: introduce `name` in MEntity according to it's documentation

Documentation was:
 # A named and possibly documented entity in the model.
So I added the name...

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

Each MProject, MGroup and MModules represents a documentable concern.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: names and concerns in model

First commit adapt the MEntity implementation to it's documentation by moving up the `name` property.

Second commit introduce a new kind of MEntity: MConcern that will be used in documentation tools as a common interface for MProject, MGroup and MModule.

Pull-Request: #522
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>