Fixing QQuickWebEngineView.loadingChanged public API test case

Rename QQuickWebEngineView.loadingStateChanged signal name to
QQuickWebEngineView.loadingChanged.

Change-Id: I0e01034f7b2eea57f5211ff0d8258d9f27d278f0
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>