Add QQuick API for intercepting navigation requests

Add missing navigationRequested API to be able to intercept
navigation requests. This is useful for ignoring requests
for example in kiosk-like applications that want to restrinct
navigation to a specific url or domain, or want to disable
specific types of navigation requests (e.g. reloading, clicking
links, form submissions).

Change-Id: Ie375e635a3c3566527972d05f5d99b39489c5ca8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>