Fixed issue #7224: comparison of dates (partial fix)

Dev: LEMval() now returns answers to date questions in yyyy-mm-dd HH:MM
Dev: format, independent of the date format that is chosen for the
question
Dev: or the particular survey. Thus, comparisons between different date
Dev: questions are possible (e.g. relevance or validation equations
Dev: "arrivaldate > departuredate") even when one question is in
Dev: dd.mm.yy and the other in mm/dd/yyyy HH:MM format
Dev: (same or previous page).
Dev: qanda_helper.php: min/max logic had to be changed accordingly
Dev: and is less complicated now.
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.