Problem: customizing access to certain BPMN functionality

[yrashk]

Currently, it's not possible to limit workflows access to any standard BPMN element. However, in some cases it might be very useful. For example, I personally consider scriptTask to be a case that might require restriction, either from security perspective (running arbitrary, even if sandboxed, code) or from the point of sense of utility (I tend to think that scriptTask is an escape hatch and can conceal too much logic that ought to be represented in BPMN elements instead)

Possible solution: make it possible to define own handler for elements at the Model level

Implementation-wise, this can be something like doing use BXPE.Engine.Model which populates all code there is now but allows overriding its add_* functions to block access. This way, the new module can be used as a drop-in replacement for BPXE.Engine.Model.