Subprocesses should have start event

[barmac]

Is your feature request related to a problem? Please describe.

When I drag a subprocess from the palette, it already contains a start event which is very convenient. It's not the case with subprocesses appended via this library.

Screen.Recording.2024-02-01.at.15.13.35.mov

Describe the solution you'd like

Given I append or create:

• a subprocess
• a transaction
• an event subprocess

it should contain a start event from the beginning.

Describe alternatives you've considered

Do nothing.

Additional context

Discovered in a meeting with @falko, @NPDeehan, and @marco-as-lopes