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/Makefile.am

@@ -30,6 +30,7 @@ dist_lisp_LISP=			\
 	mu4e-headers.el		\
 	mu4e-icalendar.el	\
 	mu4e-lists.el		\
+	mu4e-helpers.el		\
 	mu4e-main.el		\
 	mu4e-mark.el		\
 	mu4e-message.el		\