In this PR:

• Add a children property to the CalendarResource interface
• In the Event Calendar side bar, render only the top level resources and change the behavior of the selector so that events in descendant resources are also rendered.
• In the Timeline view, render the row of the children below its parent (no need to think about the DOM structure and the collapse/expand for now, we'll see what the virtualizer gives us out of the box).
• In the editing UI, flatten the list of the resources (show in this order: 1, 1-1, 1-2, 2, 2-1, ...)

Not in this PR:

• Alow to store the resource children in another property (we will need a prop like getResourceChildren)
• Allow to collapse / expand a resource's children from de Timeline
• Render the children resources in the Event Calendar sidebar
• Allow to create a child resource from the Timeline
• Allow to prevent assignating events to the parent resources