Add read-only flag for columns

As the nuxeo-spreadsheet's javascript code is now part of the Nuxeo Web UI in LTS 2021 (and therefore vulcanized), code overwrites done by a contributor in 10.10 is not anymore possible in LTS 2021.

A contributor implemented a feature allowing them to set a column read-only in 10.10.

They modified:

• https://github.com/nuxeo/nuxeo-platform-spreadsheet/blob/10.10/nuxeo-platform-spreadsheet-web/src/main/js/app/ui/spreadsheet.js to expose the read-only flag
• and the element nuxeo-spreadsheet-button to take into account the new read-only flag

Integrating this feature would be a nice bonus for the spreadsheet editor and make the upgrade to LTS 2021 less painful.