Quick: Add Favicon API

Adds a favicon API modelled after the WebKit2 QQuickWebView API, but
using an http(s) URL instead of a custom protocol, because there's no
icondatabase yet.

The icon URL lingers even when a new load is committed, until the load
finishes. It might be more prudent to clear the icon when committing a
new load, but I opted to let the app take care of that detail if
desired. Many browsers show a spinner instead of the favicon while
loading, for example.

There's no widget API implementation for favicons yet, because that API
only makes sense if we have a full-fledged icon database (case in
point: QWebEngineSettings::iconForUrl()).

Change-Id: I1e7b85104c80de2ae46a5fe9a273104d43a5c71f
Reviewed-by: Andras Becsi <andras.becsi@digia.com>