Close all tabs sets new arg for setPreviousFiles

closeAllTabs now passes 2nd arg to closeTab, setting true if we've got
more tabs to close, false if not. The dontSetPV arg determins if we
should setPreviousFiles or not. This means we only set this once all
tabs have been closed.
Also fixed issue with setting previousFiles, wasn't setting on 'CLEAR',
ie, no tabs open. Now sets a blank to handle this.