Implement QWebEnginePage::findText

A few changes to the API:
- Return the success result asynchronously.
- FindWrapsAroundDocument and HighlightAllOccurrences are enabled by
  defaults and cannot be disabled.
- Found text isn't updating the selection on the page like QtWebKit
  did, but triggers a separate state not available. A find count and
  current index could be exposed, but isn't in this case to keep the
  API delta lower.

This also adds the possibility to pass bool results through the
CallbackDirectory and add a new tst_QWebEnginePage::findTextResult
test since the old test relied on the selection to be updated when
the searched text is found.

Change-Id: I8189b5aea8d832df183c6c1ae03e3f08198a9c45
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>