Freeze break: gnome-shell: clarify confusion with old logged messages

[Owen Taylor <otaylor redhat com>]

So, say you were talking to someone last Wednesday in Empathy,
then they ping you now and you two start talking, you'll see in the
GNOME Shell notification bubble.

 JD: OK, I'm heading home now
 me: See you
 JD: You around?
                                   Sent at 15:50PM on Monday
 me: Yep
 JD: Hey, you interested in getting lunch?

Which is pretty confusing for something that's a headline feature
of GNOME 3 .... the first message is before the timestamp, and the
timestamp is the timestamp of right now - there is no indication that
the old messages are old. This regressed when we added retrieval of
logged messages, before we'd just display

 JD: You around?
 me: Yep
 JD: Hey, you interested in getting lunch?

With no timestamp unless there was a pause in the current conversation.
What we'd like to see is:

 JD: OK, I'm heading home now
 me: See you
                            Sent on Wednesday, March 23
 JD: You around?
 me: Yep
 JD: Hey, you interested in getting lunch?

https://bugzilla.gnome.org/show_bug.cgi?id=645609 has patches that form
a pretty effective fix for this, two from Jasper St. Pierre, and one
from me.

 - Impact on string freeze - none - the first two patches don't add
   strings. The third patch carefully recycles strings from the
   calendar to avoid adding anything. (Accepting a slightly worse
   appearance - it will just say "Wednesday, March 23" instead
   of "Sent on Wednesday, March 23".

   I did a test run of 'make update-po' to make sure no unexpected
   strings appeared in the .po files.

 - Impact on UI freeze -  My strong assumption is that no marketing
   materials or docs will have featured screenshots of the confusing
   appearance described at the beginning of this mail.

 - Stability - patches are all straightforward, no concerns
   on my part.

So, would like to get these in. (*Almost* done with the series of
freeze break requests - about two more on my list. Sorry about the
number here - trying to get as much as possible in before 2.91.3
and hopefully have nothing or almost nothing for 3.0.0)

- Owen