[FEAT] Consider boundary events in the sort/layout algorithm

[tbouffard]

Is your feature request related to a problem? Please describe.
Currently, the boundary events are considered as a isolated elements for the layout instead of being attached to the activity.

Describe the solution you'd like
A Boundary events should be attached to its parent activity

Additional context
Example with A.3.0.bpmn generated with PR #52 (the escalation boundary event is not displayed here as the screenshot has been done with bpmn-visualization@0.3.0 which does not support it)

original layout