mu4e-helpers: split off from mu4e-utils

Create a new mu4e-helpers, which will usurp some of the parts mu4e-utils that do
not depend on other parts of mu4e.

mu4e/meson.build

@@ -35,6 +35,7 @@ mu4e_srcs=[
   'mu4e-draft.el',
   'mu4e.el',
   'mu4e-headers.el',
+  'mu4e-helpers.el',
   'mu4e-icalendar.el',
   'mu4e-lists.el',
   'mu4e-main.el',