Manage readonly state fields on calendar views
Scenario:
- Create a production and change to state "assign"
- The Planned Date field in assign state is readonly in form view [1]
- Go to calendar view. You could drag-and-drop the production to other day -> change the planned date [2]
TO fix 3.4, 3.6, 3.8 and 4.2.
[1] https://bitbucket.org/tryton/production/src/923ba8bdea45e99b35c68932cfbced4e4e61951e/production.py?at=default&fileviewer=file-view-default#production.py-43
[2] https://bitbucket.org/tryton/production/src/923ba8bdea45e99b35c68932cfbced4e4e61951e/view/production_calendar.xml?at=default&fileviewer=file-view-default#production_calendar.xml-4