A new major version. Not so much has changed since the last of the 1.12
series, 1.12.15, but a bunch of small fixes and:

- updated requirements:
   - require C++20
   - require meson 1.3.2
   - require glib & friend: 2.80
   - require xapian 1.4.22
    - emacs 28.1 (for mu4e)
- warn for deprecated guile (use scm)
- a number of code-cleanups since we can use C++20 now

- indexing: substantial speed-up of the clean-up phase

- scm: support the --eval command-line option

- mu4e: improve message rendering; get rid of some the unnecessary
  body-part mime indication noise

- mu4e: allow including received patches when replying, see
    mu4e-compose-reply-include-mime-types

NEWS.org has more details.

Use mm-pipe-part rather then gnus-article-pipe-part since we no longer
have the gnus buffer available.

Don't switch buffers for background searches, that might show up in the
wrong frame.

This hopefully helps for #2766.

When there are raw utf-8 headers.

Should help for #2722.

mu4e-compose-reply-include-mime-types specifies the mime-types for
message attachments that should be included in replies, with the default
set to "text/x-patch" -- i.e., patches are included in replies, so you
can comment on them.

As mentioned in #2896.

Instead of the hacky "rm -rf"

Remove mu4e--view-in-headers context and the functions that use it;
instead ensure that the functions (in mu4e-headers) handle any context
switching if needed.

This allows for removing a lot of mu4e-view-... wrappers for
mu4e-headers functions. function aliases are provided for backward
compat.