Day navigator widget: setDates()

Day navigator widget: setDates()
- change logic of setting mCurDate: if it's outside the conference range, set it to mStartDate
  (and not to mEndDate when it's "greater") -- when going to an earlier
  conference, starting on the last day doesn't really make sense
- update() the widget after changing dates.
  this might be a bit expensive but it ensure that the displayed date is
  what we want, and since there are many day navigator widgets there's
  probably no single other place

Hopefully closes #36.