mu4e: Place provide forms in their own sections

Use "_" as the title of that section so that it is less distracting
when sections are collapsed to get an overview of the library.

Using a separate section is useful because it reduces the risk of
accidentally into the middle of a library.

mu4e/mu4e-speedbar.el

@@ -129,5 +129,6 @@
 (defun mu4e-headers-speedbar-buttons (buffer) (mu4e-speedbar-buttons buffer))
 (defun mu4e-view-speedbar-buttons (buffer) (mu4e-speedbar-buttons buffer))
 
+;;; _
 (provide 'mu4e-speedbar)
 ;;; mu4e-speedbar.el ends here