Wrong display for mage metadata date, time and datetime fields about zenphoto

[acrylian]

We choose to change the date time format options and also introduce new datetime format constants for more flexibility:

• DATE_FORMAT: Formerly this confusingly could have been a date or datetime format or a preffered localized format. This is now always the date format only as the name suggests.
• TIME_FORMAT (new): Always the time format
• DATETIME_FORMAT (new): Always the datetime format combined
• DATETIME_DISPLAYFORMAT (new): Use this instead of formerly DATE_FORMAT now. This has the date format as defined and also removes the time if disabled for display. Custom themes and plugins may need to update a few things if they used the DATE_FORMAT constant directly. Nothing will break though but you will only get the date alone. All standard template functions have been updated to use the new constant otherwise.

Metadata display fixes will follow later.

[acrylian]

The display is now fixed and I also renamed some field label to make the date vs time vs date and time more clear (translation updates needed)