New feature #8524: implements min/max date in EM

Dev: implements min/max date in EM (so dates are validated and
Dev: the correct help messages appear). Adv. attributes min/maxdate
Dev: in date/time question supports yyyy, yyyy-mm-dd,
Dev: questioncode (another date question) and expressions
Dev: (eg. date(Y-m-d H:i, strtotime(birthdate)+60*60*24*365*18).
Fixed issue #8484: expressions in min/max date referring to questions on same page

Dev: Expressions in adv. attribute min/maxdate also work for the date
picker
Dev: if variables in the expression refer to questions on the same page.