Add a customisation point for widget permissions and fix amnesia issues

The "remember my selection" option wasn't working because the `missing` set still included the approved permission. Solution: remove it from `missing`.

The customisation point is to allow forks to change which widgets get which additional capabilities dependent on their own rules.

src/customisations/WidgetPermissions.ts

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2020 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Populate this class with the details of your customisations when copying it.
+import { Capability, Widget } from "matrix-widget-api";
+
+/**
+ * Approves the widget for capabilities that it requested, if any can be
+ * approved. Typically this will be used to give certain widgets capabilities
+ * without having to prompt the user to approve them. This cannot reject
+ * capabilities that Element will be automatically granting, such as the
+ * ability for Jitsi widgets to stay on screen - those will be approved
+ * regardless.
+ * @param {Widget} widget The widget to approve capabilities for.
+ * @param {Set<Capability>} requestedCapabilities The capabilities the widget requested.
+ * @returns {Set<Capability>} Resolves to the capabilities that are approved for use
+ * by the widget. If none are approved, this should return an empty Set.
+ */
+async function preapproveCapabilities(widget: Widget, requestedCapabilities: Set<Capability>): Promise<Set<Capability>> {
+    return new Set(); // no additional capabilities approved
+}
+
+// This interface summarises all available customisation points and also marks
+// them all as optional. This allows customisers to only define and export the
+// customisations they need while still maintaining type safety.
+export interface IWidgetPermissionCustomisations {
+    preapproveCapabilities?: typeof preapproveCapabilities;
+}
+
+// A real customisation module will define and export one or more of the
+// customisation points that make up the interface above.
+export const WidgetPermissionCustomisations: IWidgetPermissionCustomisations = {};