* calc/calc.el (calc-gregorian-switch): In menu, put dates before regions.

* calc/calc.el (calc-gregorian-switch): In menu, put dates before regions.

This is easier to follow, lines up better in the menu, and lets us
coalesce regions that switch at the same time.  Give country
names, not "Vatican", as that's better for non-expert users.  Use
names that are stable between the date of switch and now, e.g.,
Bohemia and Moravia (which existed then and now) and not
Czechoslovakia (which didn't exist then and doesn't exist now).
What is now the U.S. mostly did not switch at the same time as
Britain, so omit the U.S.  Correct spelling of "Britain".
Catholic Switzerland was too much of a mess, so omit it.