Add support for date of death

[bentley]

The DEATHDATE property is described in RFC 6474.

Ideally this would be viewable and editable in the same manner as BDAY and ANNIVERSARY.

[zeugor]

At the same time we add the death date, we might consider to do the following suggestion:
For email fields it is already implemented a custom type where we have to also enter the custom email type label.
Could we do something similar for the dates? Currently we can enter only the date types: anniversary, birthday and other.
In my particular case, right now I would like to label a date with 'wedding'.

[RokeJulianLockhart]

@zeugor, you should probably ask that separately, because that value might not be supported by any codified standard (vCard, etcetera).

If you ask, maybe ask for custom labels, but with presets, such as "Wedding".