Modal: remove support for onFinished callback (#29852)

* 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

test/unit-tests/Modal-test.ts

@@ -9,6 +9,7 @@ Please see LICENSE files in the repository root for full details.
 import Modal from "../../src/Modal";
 import QuestionDialog from "../../src/components/views/dialogs/QuestionDialog";
 import defaultDispatcher from "../../src/dispatcher/dispatcher";
+import { flushPromises } from "../test-utils";
 
 describe("Modal", () => {
     test("forceCloseAllModals should close all open modals", () => {
@@ -23,7 +24,7 @@ describe("Modal", () => {
         expect(Modal.hasDialogs()).toBe(false);
     });
 
-    test("open modals should be closed on logout", () => {
+    test("open modals should be closed on logout", async () => {
         const modal1OnFinished = jest.fn();
         const modal2OnFinished = jest.fn();
 
@@ -31,18 +32,18 @@ describe("Modal", () => {
             title: "Test dialog 1",
             description: "This is a test dialog",
             button: "Word",
-            onFinished: modal1OnFinished,
-        });
+        }).finished.then(modal1OnFinished);
 
         Modal.createDialog(QuestionDialog, {
             title: "Test dialog 2",
             description: "This is a test dialog",
             button: "Word",
-            onFinished: modal2OnFinished,
-        });
+        }).finished.then(modal2OnFinished);
 
         defaultDispatcher.dispatch({ action: "logout" }, true);
 
+        await flushPromises();
+
         expect(modal1OnFinished).toHaveBeenCalled();
         expect(modal2OnFinished).toHaveBeenCalled();
     });