Approach
====

Without this change, the `Select` component doesn't know the `Options`
(items in the internal `List` component) after `onMount`. So it can't tell
which `Option`s are added/removed.

To fix the root cause, I created a new events to `List`: `SMUIList:mountItem`
and `SMUIList:unmountItem`, which tells the parent when some of its children
are added or removed. Then, I implemented an event handler of `Select` for the
new events to update the internal list and call `layoutOptions`. `layoutOptions`
should always be called whenever the options are updated

Another Option
====

The change might be simpler adding `bind:accessor={list}` to the `List`. But I
didn't choose it because `List` seems to want to hide its `accessor` as the
implementation details. I'll rewrite if you prefer.

Ref: hperrin#538.

Ref: https://discord.com/channels/833139170703704115/1228040959670091787