Implement loadHtml function for QQuickWebEngineView

Implement unreachableUrl support for WebContentsAdapter::setContent
and add loadProgressAfterLoadHtml QML test case.

Change-Id: I2d0d7834cbbf8f508de4fa9cb72240f2ac6ebf83
Reviewed-by: Andras Becsi <andras.becsi@digia.com>