Fix Gmail complaining window creation was prevented by a popup blocker (PR #603)

By changing incorrect window `guest` property to `newGuest`. See
https://github.com/electron/electron/blob/master/docs/api/web-contents.md#event-new-window

> Calling `event.preventDefault()` will prevent Electron from 
> automatically creating a new BrowserWindow. If you call
> `event.preventDefault()` and manually create a new BrowserWindow
> then you must set `event.newGuest` to reference the new BrowserWindow
> instance, failing to do so may result in unexpected behavior.

app/src/components/mainWindow/mainWindow.js

@@ -199,7 +199,7 @@ function createMainWindow(inpOptions, onAppQuit, setDockBadge) {
       return;
     }
     // eslint-disable-next-line no-param-reassign
-    event.guest = createNewWindow(urlToGo);
+    event.newGuest = createNewWindow(urlToGo);
   };
 
   const sendParamsOnDidFinishLoad = (window) => {