Fix a crash in WebEngineSettings when opening a new window

A new window means that the QWebEnginePage gets its WebContentsAdapter
swapped and that the pointer in WebEngineSettings must be updated.

Do the WebContentsAdapter-to-WebEngineSettings binding in
WebContentsAdapter::initialize to cover both cases.

This also refactors the way that QWebEngineSettings is created by
removing the need to pass a QWebEngineSettingsPrivate instance to be
adopted, and also move the global settings construction logic in
the singleton accessor instead of relying on the fact that it uses
a different contructor.

Change-Id: I6f8a2ed1407a4b25f9898526db9432721c354ddf
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>