stable release 1.12.7

- many documentation fixes
- mu4e: fix some mu4e-compose-captured-message corner case (#2745)
- mu4e: fix 'user-agent' field (#2750)
- few tiny tweaks

However biggest change is regarding mu's "threadiness" with respect to
Xapian. For a while, some people, ran into Xapian database
corruptions (I've seen it very occasionally, some people more often);
issue #2706.

I don't know the precise cause, but it seems to be related to using
threads with Xapian; the mu code in this area hasn't changed much, so
perhaps newer Xapian versions are less forgiving? Anyway, this is all
speculation. Database corruption with mu is not catastrophic, since you
can always rebuild it without data-loss; however, it's still annoying
and we'd obviously want to avoid that.

1.12.7 includes a change to make the database access single-threaded
again, which seems to help for this problem. We're really returning to
the behavior some older versions of mu, with the difference that now
you'll get a message in mu4e

Cannot handle command while indexing, please retry later.

Hope that doesn't inconvenience people too much. Might have a deeper
look into when I have some more time and not in the 1.12.x series.

(there's a meson flag to keep the existing behavior, see the commit
message).
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