Instead of iterating over each of the entities in a grouping to find out
which groups should be displayed based on the selector's policy, the
selector can directly keep its `selectedGroups` attribute in sync with
the index.
It was broken by the previous refactor of the entities panel, which no
longer triggers the `watch` callback on the upstream `entityGroups`.
The new approach listens for entity updates on the frontend bus and
dynamically creates the entity groupings in `selectedGroups` if they are
missing.