Problem
---------
When you go in the settings -> layout (debug mode) -> select external
layout --> preview document a traceback appears due to infinite looping
in the view definitions.

Solution
---------
This field is highly technical and should not be fiddled with by users.
For such reason, we set it as `readonly`.

task-3479152

closes odoo/odoo#133790

Signed-off-by: Quentin De Paoli <qdp@odoo.com>