Fix StatusbarProgressWidget::slotProgressItemCompleted() method

The patch provides fix for case when StatusbarProgressWidget displays single status item which use busy indicator. Such items can be provided for example by jobs which don't know progress information during run. For this case old version displays progress bar in still busy state for "final" 5 seconds after the item is finished. Such behavior is very misleading - it's impossible to understand the state (run/finish) of progress item. New version fix this by changing the state of progress bar to 100% after item finishing, which is more clear for user.