Split out the Widgets and QtQuick integration

This is the first step to making proper Qt Modules out of QtWebEngine.
The Widgets integration becomes a proper C++ Qt Module while we make
the QtQuick side a QML plugin for now (could probably be promoted if
the need arises).

Code-wise, this means the introduction of a WebContentsAdapterClient
interface that is subclassed by the private implementation of our API
classes for delegation of things that are UI specific. Functionality
from WebContents and the like is exposed via the WebContentsAdapter.

Change-Id: I4ca3395b9fe8502a24e36002cfd5af44067bb6e8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>