Back/forward history menus now won't show duplicated entries and also
navigating with back/forward on pages won't leave you on the same page
as you are actually visiting.
Though this bug was appearing rarely, it was annoying.
This is not normal QtWebKit behaviour, in standard implementation it is
working just fine, but QupZilla is using QWebPage::extension error
pages, and when this error page appears in frame on page (for example in
<iframe>) it duplicates entry in history.
