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>