Mark AppEventEmitter as disposable

[robertbrignull]

Makes AppEventEmitter implement Disposable. This class is mirroring the VS Code EventEmitter class, and that does have a dispose method.

This means in places where we use app.createEventEmitter we can track the object and dispose of it, which helps mirror the places where we use EventEmitter directly and we already do this.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[robertbrignull]

Thanks for reviewing. I just realised I forgot to also make sure that DbManager is disposed of, so I've just pushed that. I think that change is small enough and in keeping with the rest of the PR so it doesn't invalidate the approval.