New stable release 1.12.14

Nothing too spectacular, but 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.

See NEWS.org for further details.