Cleanup the QtWebEngineWidgets public headers and API

Headers were left intact to leave a trace of the evolution compared
to the QtWebKit API and to make it easier to work until we had a
basic subset of the API implemented.

With the upcoming release, this patch removes this convenience in the
aim of starting polishing the headers and the documentation for the
upcoming release.

Change-Id: Iae436b4ec041d771a7002575e122835802bc9f3e
Reviewed-by: Michael Bruning <michael.bruning@digia.com>