Do away without QWebEngineWidgets::initialize

If we consider the plugin scenario is unlikely and decide it's
unsupported for widgets, we can simplify our tests and examples
a bit on this front.

Change-Id: Idc96032c127b4ee74fb5c7b3d2cdfdf99c3a722e
Reviewed-by: Andras Becsi <andras.becsi@digia.com>