Add api to get the favicon URL

Adds one of the missing pieces of the QWebFrame and QWebView APIs.
Unlike the QtWebKit version this only fetches the favicon URL, and not
the icon. This is because we do not want to implement an icon database,
and that the icon would be loaded asynchronous anyway, bringing no
guarantee to be a valid icon/image yet.

Change-Id: I227311ae3676044da850e687b82bee752b5079c8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>