Abort findText when destroying QWebEnginePage

In the destructor of QWebEnginePage and QQuickWebEngineView, we
now call adapter->stopFinding() to suppress callbacks that would
cause segmentation faults if a findText request is still active
at that point.

Task-number: QTBUG-58563
Change-Id: I9d5d8407ee6c28cd76bf6ecf0b62808394e151cd
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>