Expose DidFinishNavigation()/HasCommitted() through the public API

The current API only exposes the start of the navigation and the end of the load.

Qt users cannot know when the navigation finished and the page actually starts rendering.
This is impratcical when the surrounding UI need to be updated based on the current visible page:

• loadStarted() is too early to update UI
• loadFinished() is too late to update the UI
• urlChanged() is right on time, but is not emitted during a reload and is emitted when a web app changes its URL without navigation

Right now, I need to combine all 3 of them to kind of get what I want.

It would be much simpler if:

• QWebEnginePage:::loadingChanged() was also emitted when DidFinishNavigation() is called
• QWebEngineLoadingInfo had a hasCommited property