Allow for custom dates (X-ABDATE, X-ABLABEL)

[call-me-matt]

related to #407 , #926

I have many custom dates in my contacts (wedding day, birthday of kids, ...) and they are working fine, also with my calendar.
Unfortunately it is not possible to edit these custom dates with SimpleContacts and I always have to switch to another contacts app for this.
It would be great if support for X-ABDATE with X-ABLABEL could be added to SimpleContacts.

Example:

BEGIN:VCARD
VERSION:3.0
PRODID:-//Sabre//Sabre VObject 4.5.4//EN
UID:6acfa7d3-d1dc-4750-9dce-908bff3232d2
FN:Richard Stallmann
N:Stallmann;Richard;;;
BDAY:19530316
ITEM1.X-ABLABEL:GNU Manifesto
ITEM1.X-ABDATE:19850101
REV:20240328T061445Z
END:VCARD