Fix QWebEngineView to page reattachment.

This fixes the crash in tst_QWebEngineView::reusePage.
Also add a test to check the case where show() would react incorrectly.

Change-Id: I40247c7c225d74b26675b6a7fa5ff1f06d3bb3e6
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>