gtd-task: add support for completion date
save completion date when a task is marked as complete and unset when unmarked as complete.
Issue Link: https://gitlab.gnome.org/GNOME/gnome-todo/issues/186
Edited by Rohit Kaushik
save completion date when a task is marked as complete and unset when unmarked as complete.
Issue Link: https://gitlab.gnome.org/GNOME/gnome-todo/issues/186