* Fix up type for `finished` result of Modal
The `finished` promise can be called with an empty array, for example if the
dialog is closed by a background click. This was not correctly represented in
the typing. Fix that, and add some documentation while we're at it.
* Type fixes to onFinished callbacks from Modal
These can all be called with zero arguments, despite what the type annotations
may say, so mark them accordingly.
* Remove uses of Modal `onFinished` property
... because it is confusing.
Instead, use the `finished` promise returned by `createDialog`.
* Modal: remove support for now-unused `onFinished` prop
* StopGapWidgetDriver: use `await` instead of promise chaining
* Fix up unit tests
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