electron-bridge: Implement electron bridge.

This PR adds a bridge to communicate with the webapp in real time. As of now, the bridge listens for following events -

* When realm name changes
* When realm icon changes
* When the unread count changes

Partially fixes #425.

app/renderer/js/electron-bridge.js

@@ -0,0 +1,39 @@
+const events = require('events');
+const { ipcRenderer } = require('electron');
+
+// we have and will have some non camelcase stuff
+// while working with zulip so just turning the rule off
+// for the whole file.
+/* eslint-disable camelcase */
+class ElectronBridge extends events {
+	send_event(...args) {
+		this.emit(...args);
+	}
+
+	on_event(...args) {
+		this.on(...args);
+	}
+}
+
+const electron_bridge = new ElectronBridge();
+
+electron_bridge.on('total_unread_count', (...args) => {
+	ipcRenderer.send('unread-count', ...args);
+});
+
+electron_bridge.on('realm_name', (...args) => {
+	ipcRenderer.send('realm-name-changed', ...args);
+});
+
+electron_bridge.on('realm_icon_url', iconURL => {
+	const serverURL = location.origin;
+	iconURL = iconURL.includes('http') ? iconURL : `${serverURL}${iconURL}`;
+	ipcRenderer.send('realm-icon-changed', serverURL, iconURL);
+});
+
+// this follows node's idiomatic implementation of event
+// emitters to make event handling more simpler instead of using
+// functions zulip side will emit event using ElectronBrigde.send_event
+// which is alias of .emit and on this side we can handle the data by adding
+// a listener for the event.
+module.exports = electron_bridge;