[fix] Property access for Modal Executor

[ibelar]

Allow private property access.
Useful when a special form is needed in Modal Executor.

• When model action uses Arguments you can safely return using $this->getStep() when the customs form is for field step only;
• Get previous step data access using getActionData($step)

[PhilippGrashoff]

Talking about private, wdyt about #1537? I think putting a lot of private in our code makes it less extendable and customizable. IMO private should only be used over protected when there is a very good reason for it.

[DarkSide666]

LGTM