Stunner - Improve UX for Event's Data IO properties

Some of the BPMN events provide support for the Data IO property assignments. It's actually handled by using the same editor (widget) as for tasks, but the allowed data IO properties for events are more restrictive than when assigning into tasks:

• Intermediate throwing events and end events only support data inputs, intermediate catching events and start events only data outputs (note that this is already enforced since the data IO popup will only show the relevant type)
• Most events only support one data item to be mapped (typically using a hardcoded parameter name).

Recommendation would be to not rely on the Data IO popup, but consider to only add a Target dropdown (for outputs) or Source dropdown (for inputs) to the properties panel directly, as this is the only information that needs to be provided.

Reported by KrisVerlaenen