Replace countRoomsWithNotif with a dedicated NotificationState

Fixes https://github.com/vector-im/riot-web/issues/14694

Instead of spending 10-1000ms in a function iterating over a whole lot of room events, we can use our cached state from the Notification State Store. 

This commit sets up a structure that could be applied to communities in the TagPanel too, as that could probably use a similar optimization.

This reduces the updateStatusIndicator() time to just 4ms on average.

src/stores/notifications/SummarizedNotificationState.ts

@@ -0,0 +1,62 @@
+/*
+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.
+*/
+
+import { NotificationColor } from "./NotificationColor";
+import { NotificationState } from "./NotificationState";
+
+/**
+ * Summarizes a number of states into a unique snapshot. To populate, call
+ * the add() function with the notification states to be included.
+ *
+ * Useful for community notification counts, global notification counts, etc.
+ */
+export class SummarizedNotificationState extends NotificationState {
+    private _totalStatesWithUnread = 0;
+
+    constructor() {
+        super();
+        this._symbol = null;
+        this._count = 0;
+        this._color = NotificationColor.None;
+    }
+
+    public get numUnreadStates(): number {
+        return this._totalStatesWithUnread;
+    }
+
+    /**
+     * Append a notification state to this snapshot, taking the loudest NotificationColor
+     * of the two. By default this will not adopt the symbol of the other notification
+     * state to prevent the count from being lost in typical usage.
+     * @param other The other notification state to append.
+     * @param includeSymbol If true, the notification state's symbol will be taken if one
+     * is present.
+     */
+    public add(other: NotificationState, includeSymbol = false) {
+        if (other.symbol && includeSymbol) {
+            this._symbol = other.symbol;
+        }
+        if (other.count) {
+            this._count += other.count;
+        }
+        if (other.color > this.color) {
+            this._color = other.color;
+        }
+        if (other.hasUnreadCount) {
+            this._totalStatesWithUnread++;
+        }
+    }
+}