Basic font settings.

Include a default value getter in the store in order to make the
default value easy to work with.

src/settings/controllers/FontSizeController.js

@@ -0,0 +1,33 @@
+/*
+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 SettingController from "./SettingController";
+import dis from "../../dispatcher";
+
+export default class FontSizeController extends SettingController {
+
+    constructor() {
+        super()
+    }
+
+    onChange(level, roomId, newValue) {
+        // Dispatch font size change so that everything open responds to the change.
+        dis.dispatch({
+            action: "update-font-size",
+            size: newValue,
+        });
+    }
+}