Remove QQuickWebEngineSettingsPrivate object's pointer from the global
setting list before the object is destructed.

Change-Id: I1cca6bf8aa2438903692d9e2ef863c8fc543adbd
Reviewed-by: Andras Becsi <andras.becsi@digia.com>

This is required so that embedders can implement custom error pages,
and is also needed for tests, where error-page loading has side-effects
on subsequent page loads.

Change-Id: I5ae276a58864a2fa9d7b608bea3340b482a92f4e
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>

Expose settings in experimental for now.
Simply use the global/per-view mechanism as a group level
in between seems like overkill.

Change-Id: I7610e9c7765ccb41674702acdd24a52b87716da8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>