Add loadProgress API

This exposes loadProgress in both widget and quick webengineviews.
However, the progress will not change until we get an upstream change
in Chromium where the content LoadProgressChanged API is exposed to all
ports, not just Android. The upstream change is
https://src.chromium.org/viewvc/chrome?revision=221010&view=revision

Once we get that change, you'll see the widget example browser start to
paint a blue progress rectangle in the background of the URL bar. Also,
a progress bar was added to the quicknanobrowser, but it will be stuck
at 0 for now.

Change-Id: Icbaa01b86c013e0052b3abb7672c38e57128f44a
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>