New feature #7810: Implement max/min date for date question

DEV: implemented min/max dates for datepicker in Date question
DEV: supported formats/dates:
DEV: - fixed dates in format YYYY-MM-DD (e.g. 2013-08-24)
DEV: - {qcode}: reference to another datefield
DEV: -      supported on same (dynamic, JS) and on previous pages (PHP)
DEV: -      honors survey date format and date formats in
DEV: -      advanced attributes of a date question (mixing date
DEV: -      formats in one survey should be no problem)
DEV: - {expressions}: only on following page
DEV: -      Things like date from previous answer+3 days as min
DEV: -      for another question is possible using eg.
DEV: -      {date("Y-m-d",strtotime(departure)+3*60*60*24)}
DEV: -      on a following page
DEV:
DEV: I renamed the dropdown_dates_year_min/max fields
DEV: so the name mirrors their extended use. The new fields still
DEV: take fixed years in format YYYY for backward compatibility.
DEV: (field names/values are converted when importing old lss files).
DEV:
DEV: This also fixes #7981 (bug in jquery UI) because we do not use
DEV: the yearrange function of the datepicker anymore,
DEV: which is affected by this bug.
DEV:
DEV: Tested in FF and Chrome