Implement the event trigger command to initialize events

[Palbolsky]

Description

This PR adds the event trigger command (also known as the start command) to initialize events.

Closes #636
Closes #765

Tests to perform

• The user can add the start command in the event
• The user can edit the trigger in the node
• The user can edit the trigger and the execution order in the editor
• When the user deletes a start command, check that the executions orders are recalculated correctly so that they are 1, 2, 3, etc.
• When the user edit a execution order, check that the executions orders are recalculated correctly: there must not be two identical executions orders

Screenshot