Stop disabled QWebEngineView widget from processing input events.

If a QWebEngineView is disabled, input events are still forwarded to
Chromium, thus allowing the user to interact with a web page.

Fix consists in stopping the forwarding of input events in the generic
event handler, just like QWidget::event() does.

Change-Id: Ie822d1f3d640840569a282223d76749686cf3419
Reviewed-by: Michal Klocek <michal.klocek@theqtcompany.com>