add "show sessions" to the history query options, so that we can disable it.
by default, disable sessions for the history-on-places sidebar (as we are initially going for Firefox 2.0 parity on top of places.)
To enable sessions, toggle the hidden "browser.history.showSessions" boolean pref.
Thanks to Dan Mills <thunder@mozilla.com> for the fix.
r=sspitzer,gavin
git-svn-id: svn://10.0.0.236/trunk@216769 18797224-902f-48f8-a5cc-f745e15eee43