Add support for stage concept

[brunobuzzi]

https://doc.orbeon.com/release-notes/orbeon-forms-2020.1#notable-features-and-enhancement
"The concept of stage is introduced at the database level and can be controlled with the following:"

https://doc.orbeon.com/form-builder/form-settings#formula
Read-Only: Boolean expression specifying whether the entire form is read-only (not editable).
If this field is left blank, then the form is editable.
Otherwise, it is editable only if the result of the Boolean expression is false().

https://doc.orbeon.com/form-runner/advanced/buttons-and-processes/actions-form-runner#set-workflow-stage