Allow traversing arbitrary relations in a single query

[RiscadoA]

Problem

Currently, if it isn't possible to write a Query which matches against all entities who have an ancestor with a given component.

For example, imagine you have a scene such as:

• foo (component A)
  • bar (component B)
    • baz (component B)

And you want to write a query which matches against (bar, foo) and (baz, foo), such as:

Query<const B&, Indirect<const ChildOf&>, const A&>

I.e., the ChildOf relation could be indirect

Solution

Needs design.