Added code to support multiple browser windows. Fixes issue #7

[alandmoore]

This is to address issue #7. By creating a callback that generates a new browser window and assigning it to MainWindow.browser.createWindow, the browser is able to open new windows with Ctrl+click and handle target='_blank' links.

In the spirit of this repository, I have tried to keep the code simple and minimal. A more proper implementation would subclass QWebEngineView or QWebEnginePage, but this approach is simple and works.

[SaptakBhoumik]

Approved