Display 'YYYY', 'MM' and 'DD' in date input field when value not available

from http://bugs.musicbrainz.org/ticket/2840

When a date of a track is not present, or only partially available, in the file and/or in the db, the date input field will display ' - - '. It's not for everyone as logical to know they should enter the date in a 'YYYY-MM-DD' style. Even if only the year is present there will be people at some stage who will be turning the month and day around. Or than there are those people who insist of entering it in a different style like 'DD-MM-YYYY'. To overcome these problems it would be handy to display 'YYYY', 'MM' and 'DD' for those fields where the value isn't available with maybe a light grey font colour. When the user clicks on a date part, this text can disappear and reappear when nothing is entered.