Add a ToastStore

To store toast. Rather than them being stored in the state of the
ToastContainer component, they now have a dedicated store. This mostly
fixes problems involving showing toasts when the app loaded because
we would otherwise have a race condition where something tries to
show a toast before the ToastContainer is mounted.

src/stores/ToastStore.js

@@ -0,0 +1,52 @@
+/*
+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 EventEmitter from 'events';
+
+/**
+ * Holds the active toasts
+ */
+export default class ToastStore extends EventEmitter {
+    static sharedInstance() {
+        if (!global.mx_ToastStore) global.mx_ToastStore = new ToastStore();
+        return global.mx_ToastStore;
+    }
+
+    constructor() {
+        super();
+        this._dispatcherRef = null;
+        this._toasts = [];
+    }
+
+    addOrReplaceToast(newToast) {
+        const oldIndex = this._toasts.findIndex(t => t.key === newToast.key);
+        if (oldIndex === -1) {
+            this._toasts.push(newToast);
+        } else {
+            this._toasts[oldIndex] = newToast;
+        }
+        this.emit('update');
+    }
+
+    dismissToast(key) {
+        this._toasts = this._toasts.filter(t => t.key !== key);
+        this.emit('update');
+    }
+
+    getToasts() {
+        return this._toasts;
+    }
+}