Implement always-on-screen capability for widgets

As per https://github.com/matrix-org/matrix-doc/issues/1354

This is whitelisted to only jitsi widgets for now as per comment,
mostly because any widget that we may make always-on-screen we need
to preemptively put in a PersistedElement container, which is
unnecessary for any other widget.

Apologies that this does a bunch of refactoring which could have
been split out separately: I only discovered what needed to be
refactored in the process of doing this.

Fixes https://github.com/vector-im/riot-web/issues/6984

src/stores/ActiveWidgetStore.js

@@ -0,0 +1,84 @@
+/*
+Copyright 2018 New Vector Ltd
+
+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.
+*/
+
+/**
+ * Stores information about the widgets active in the app right now:
+ *  * What widget is set to remain always-on-screen, if any
+ *    Only one widget may be 'always on screen' at any one time.
+ *  * Negotiated capabilities for active apps
+ */
+class ActiveWidgetStore {
+    constructor() {
+        this._persistentWidgetId = null;
+
+        // A list of negotiated capabilities for each widget, by ID
+        // {
+        //     widgetId: [caps...],
+        // }
+        this._capsByWidgetId = {};
+
+        // A WidgetMessaging instance for each widget ID
+        this._widgetMessagingByWidgetId = {};
+    }
+
+    setWidgetPersistence(widgetId, val) {
+        if (this._persistentWidgetId === widgetId && !val) {
+            this._persistentWidgetId = null;
+        } else if (this._persistentWidgetId !== widgetId && val) {
+            this._persistentWidgetId = widgetId;
+        }
+    }
+
+    getWidgetPersistence(widgetId) {
+        return this._persistentWidgetId === widgetId;
+    }
+
+    setWidgetCapabilities(widgetId, caps) {
+        this._capsByWidgetId[widgetId] = caps;
+    }
+
+    widgetHasCapability(widgetId, cap) {
+        return this._capsByWidgetId[widgetId] && this._capsByWidgetId[widgetId].includes(cap);
+    }
+
+    delWidgetCapabilities(widgetId) {
+        delete this._capsByWidgetId[widgetId];
+    }
+
+    setWidgetMessaging(widgetId, wm) {
+        this._widgetMessagingByWidgetId[widgetId] = wm;
+    }
+
+    getWidgetMessaging(widgetId) {
+        return this._widgetMessagingByWidgetId[widgetId];
+    }
+
+    delWidgetMessaging(widgetId) {
+        if (this._widgetMessagingByWidgetId[widgetId]) {
+            try {
+                this._widgetMessagingByWidgetId[widgetId].stop();
+            } catch (e) {
+                console.error('Failed to stop listening for widgetMessaging events', e.message);
+            }
+            delete this._widgetMessagingByWidgetId[widgetId];
+        }
+    }
+}
+
+if (global.singletonActiveWidgetStore === undefined) {
+    global.singletonActiveWidgetStore = new ActiveWidgetStore();
+}
+export default global.singletonActiveWidgetStore;