Introduce relationships between parents and children

[bartfeenstra]

Parent-child relationships can be complex, but we do not currently support any additional information about them. By introducing relationships between people, we can extend these with any necessary metadata in the future, such as whether children were adopted, and when.

We then ask ourselves if we want to define other types of relationships as well, such as between partners, friends, and coworkers.

We also care about directionality, or roles: who is parent of whom, who corresponded with whom, who adopted whom, etc.

What if we use events for relationships? Presence roles can already be used to indicate direction, e.g. the adoptee or the adoptive parent. We can therefore use them for parents and children as well. This immediately allows us to attach dates and places as well, e.g. Jen and Jane were coworkers from X until Y, at Z.

A solution to this issue may also fix #495