updated feature #7145: validation field for date/time question

DEV: added "validation" and respective tip field to the advanced
attributes
DEV: of the date/time question and changed em_manager_helper so field is
evaluated
DEV: and EM color coding is applied.
DEV: In connection with strtotime() one can for example enforce that
only dates
DEV: before "today" are entered: strtotime(self)<=strtotime("now")
Merge pull request #93 from mfaber/validate2

New feature #7145: validation field for date/time question