To reproduce the issue:
* clear your browsing history via the ui - do not close the browser
* start navigating pages, so that new items get inserted into the
* history model.
-> will crash as m_todayItem got deleted but not recreated, as it was
not NULLed.
- much faster history loading (loading history entries only when
needed - eg. expanding top level items)
- less memory usage (sidebar and manager are using the same model)
- searching is now performed on title and url
- clear private data -> clear history is now working properly
