Provide create Sub-Process from selection utility

[nikku]

Use case: A user wants to wrap a number of elements inside a new sub-process.

Solution: Provide that functionality as a BPMN modeling operation:

• Create sub-process with size = selection
• Move selection into sub-process
• Integrate as special action into menu (or somewhere else ?)

Hints

• On the hind-side, we should provide the inverse action, too.

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Built as an extension here: https://github.com/marstamm/bpmn-js-move-to-subprocess

[nikku]

Related to https://github.com/camunda/camunda-modeler/issues/523