Allow to choose salaryStepSeqId when create a SalaryStep

Comming from "The Data model Resource Book"

« Salary Grade-Step are used in entreprise that have a predefined, highly structure pay system (such as the federal government). This is done by reference to a structure pay schedule.»

Currently in OFBiz in HR in Global HR Settings, it's possible to choose gradeId but not stepSeqId and so it's not readable when you want select a Grade-Step for a EmployeePositionType or a Employment / PayHistory because it's not same as legal "Structure Pay Schedule" (like "Convention collective, salaire minimum par position" in France).

In patch propose, add a field in form and in create service use it if not empty