Date Formatting

- Some bad inefficiencies in date formatting were noticed, and when addressed,
  caused a bug to surface.  The length of a formatted date can be calculated
  from the dateformat, but was done incorrectly.  Very, very incorrectly.
- Added unit tests.
- Promoted date column-specific "countdown" size measurements up to the ColDate
  base class.  This neatly falls out from work on #1218.
- Noted a potential I18N problem in Date.cpp.