Check that all linked components are in the composition

Currently, it is easy to forget to add a component to a composition.

This results in an error about a wrong component state during connect.

The scheduler could follow all links to find all components, but slots don't know their owner. So we have no access to such a component.

We can find all inputs and outputs, and check if there are any that don't belong to a component.

We could, however, add a more sensible error message/hint if the error occurs in this particular phase.