New stable release v1.12.14

Nothing too spectacular, but a various small tweaks:

- improved guix (and other systems) compatibility: make fewer assumptions about where system programs are
- documentations improvements, in manpages and the mu4e reference manual
- labels: better messages when importing
- scm: stabilize tests. tweaks api.
- mu4e: add mu4e-view-mark-and-next
- mu4e: mu4e-dbus-mode; a new experimental minor in which mu4e exposes some of
  its data over DBus.
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