Annotations: add datetime_updated property

[hius07]

Closes #12957.

(1) datetime_updated is not exposed to UI.
(2) datetime_updated is not exported.
(3) There is no sorting by date modified.
(4) "Latest bookmark" still means the last created bookmark.

(5) @noembryo I do not add datetime_updated to new annotations.
Old annotations don't have this field, in any case you need to do something like

local datetime = item.datetime_updated or item.datetime

so no difference whether it is nil or equals to datetime.

---

This change is 

[noembryo]

I do not add datetime_updated to new annotations.
Old annotations don't have this field, in any case you need to do something like..

Do you mean that although that newly created annotations will get this key, annotations that are already created won't get it?
We're not talking about the old format annotations, right?
If so, then yes, this looks like a good way to do it, thanks.. 🙏

[hius07]

Newly created annotation (in new format, of course) doesn't get the key on creation.
So, if datetime_updated is nil, use datetime.

[noembryo]

A, OK. Thanks for the clarification.