Use isolated IPC API

This adjusts to match changes on the desktop side where we now use Electron's
context isolation feature to expose only the IPC channels and methods we
actually expect to use.

src/@types/global.d.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2020 New Vector Ltd
+Copyright 2020, 2021 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.
@@ -17,13 +17,32 @@ limitations under the License.
 import "matrix-react-sdk/src/@types/global"; // load matrix-react-sdk's type extensions first
 import type {Renderer} from "react-dom";
 
+type ElectronChannel =
+    "app_onAction" |
+    "before-quit" |
+    "check_updates" |
+    "install_update" |
+    "ipcCall" |
+    "ipcReply" |
+    "loudNotification" |
+    "preferences" |
+    "seshat" |
+    "seshatReply" |
+    "setBadgeCount" |
+    "update-downloaded" |
+    "userDownloadCompleted" |
+    "userDownloadOpen";
+
 declare global {
     interface Window {
         mxSendRageshake: (text: string, withLogs?: boolean) => void;
         matrixChat: ReturnType<Renderer>;
 
         // electron-only
-        ipcRenderer: any;
+        electron: {
+            on(channel: ElectronChannel, listener: (event: Event, ...args: any[]) => void): void;
+            send(channel: ElectronChannel, ...args: any[]): void;
+        }
 
         // opera-only
         opera: any;