Call stopFinding, when navigating away

When one searches for text on one page and then navigates away,
results for the search-term are also highlighted on the new page.
To stop this from happening, we now call adapter->stopFinding()
when we navigate away from a page.

Task-number: QTBUG-61506
Change-Id: I09e064d7788b12eae0a67c01e4f5872d94924617
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>