Don't webpack electron main, separate electron

* Don't webpack electron-main.js: no need to webpack in electron
 * Keep the electron file separate from the webapp, in their own
   'electron' directory

electron/src/electron-main.js

@@ -0,0 +1,18 @@
+// @flow
+const {app, BrowserWindow} = require('electron');
+
+let window = null;
+
+app.on('ready', () => {
+    window = new BrowserWindow({
+        icon: `${__dirname}/../../vector/img/logo.png`,
+    });
+    window.loadURL(`file://${__dirname}/../../vector/index.html`);
+    window.on('closed', () => {
+        window = null;
+    })
+});
+
+app.on('window-all-closed', () => {
+    app.quit();
+});