Add check for declaration of "file" workflow variable to validation of File Order Sources

Current Situation

• Recent releases of JS7 starting from 2.6.x force declaration of the file variable with a workflow that is used with file watching.
  • Reasons include that the workflow requires a file variable as it has to move/remove the file before completion of an order.
• The technical documentation indicates this requirement since earliest versions of JS7.
• With recent releases the Controller denies to start a workflow that does not declare the file variable. Related error messages are available from the Controller's log file.

Desired Behavior

• Users who are not familiar with the documentation might find it hard to identify the problem when no orders are started due to a missing file variable.
• The JS7 API should perform a check during validation of a File Order Source if the related workflow includes the declaration of a file variable.