fix: add required DTSTAMP property to VTODO components

RFC 5545 requires DTSTAMP for all VTODO and VEVENT components:

> The following are REQUIRED, but MUST NOT occur more than once.
>   dtstamp / uid

Without DTSTAMP, strict CalDAV servers may reject the VTODO or clients
may have trouble parsing it.

Sets DTSTAMP to current UTC time when generating iCalendar data.

Author: distefam

core/Objects/Item.vala

@@ -1192,6 +1192,7 @@ public class Objects.Item : Objects.BaseObject {
         ICal.Component ical = new ICal.Component.vtodo ();
 
         ical.set_uid (id);
+        ical.set_dtstamp (new ICal.Time.current_with_zone (ICal.Timezone.get_utc_timezone ()));
         ical.set_summary (content);
         ical.set_description (description);