Use a real web browser for OAuth authentication instead of the built-in OAuth webview

First, Google is beginning to block embedded webviews for OAuth authentication. Opening a web browser window for the authentication is also more secure from the user perspective as it makes it harder for a malicious app to spy on the user when they are entering their username and password, since they are now entering it in a completely different program.

Application implementations should now make sure they interact with the user before launching the AuthJob so that the user knows what's going on. If the app just ends up firing up the web browser and asking for user's Google credentials, they are very likely to cancel.

Finally, this gets us rid of QtWebEngine dependency in KGAPI. Yay!