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.
Unlike the other entity groupings, which are 4-layered (`grouping ->
group -> entity_id -> entity`), the grouping by ID only needs 3 layers
(`grouping -> entity_id -> entity`).
- Don't recalculate entity groups every time. Instead, keep them in sync
every time an entity is added or removed.
- Removed `computedChildren` from the entity component - no null nodes
are guaranteed to be passed now, so there's no need for another
iteration on the list of children.
- `childrenByParentId` now only looks in the scope of the entity's
children instead of searching all the entities.
There are probably more optimal ways of achieving this other than
passing a reference to the full list of entities to each of the
entities, such as running a BFS to recursively expand all the entities
within the child hierarchy of an entity.
This is needed because the entity needs to know which entities aren't
direct children, but are two or more layers down in the hierarchy, so
they should be passed to their own child entities.
Changed from `type` to `category`, which is basically the `name_plural`
attribute of the associated entity type metadata.
This allows us to define distinct entity metadata entries that we still
want to share the same grouping - for instance, `temperature_sensor`,
`humidity_sensor` and `battery` should all be grouped under `Sensors` on
the frontend.