Application::quit doesn't work on Windows Phone

Calling Qt.quit() or QApplication::quit will simply stop the application and make it unresponsive but not close it. The default action when pressing back will send the app to the background as it should on WP, though if the app has a an explicit quit action, this would case the app to hang.

As discussed on irc, we should probably implement quit() so it actually works as intended but possibly also print a warning saying this is not supported or ideal for Windows Phone