Fix leak of QSensors on QWebPage creation

The WebCore page supplements are not deleted when the page is, so we
need to track them in QWebPageAdapter and delete them there.

Task-number: QTBUG-38857
Change-Id: Ib3e41074de3bebf00f017523d28efc1c923250f3
Reviewed-by: Michael Bruning <michael.bruning@digia.com>