[Super Editor][IME][Bug]: Fixed zombie IME client when switching from one SuperEditor instance to a replacement SuperEditor instance (Resolve #2965)

From the issue ticket:

1. SuperEditor initializes
2. SuperEditor creates its IME client and connects it to the OS via SharedIme
3. The ancestor tree gets invalidated (a parent calling setState())
4. SuperEditor 2 initializes
5. SuperEditor 2 claims ownership over IME (but no connection is created because we're the same "role" and there's already a connection)
6. SuperEditor 1 disposes (no impact on SharedIme because this instance is no longer the owner)

Problem: The existing IME connection, which survives across the widget instance change, it bound to the IME client for SuperEditor 1, which is different from the IME client for SuperEditor 2. As a result, IME input keeps flowing to the defunct IME client, which belongs to the now disposed SuperEditor 1. This means the keyboard stays up, the user is typing, suggested input continues to display at the top of the keyboard, and yet nothing changes in the editor.

This problem is the result of me originally thinking that we could re-use the connection. I missed the fact that when claiming the SharedIme, and passing the client, we need to create an entirely new connection for the client, even if a connection is already open. There is no option in Flutter's API to replace one client with another, within the same connection.

One wrinkle to also remember here is that, I believe, when we open a new connection, we must tell the keyboard to show() if we want it to stay up. By default, if we don't do that, creating a new connection causes the keyboard to close.