mu4e-utils: add method to count the longest name

Consolidates the logic for looping over bookmarks and maildirs so that
we can compute the longest name.

mu4e/mu4e-utils.el

@@ -862,6 +862,11 @@ This is meant to be the exact same data structure as
                append (list key value))))
    (mu4e-maildir-shortcuts)))
 
+(defun mu4e~longest-of-maildirs-and-bookmarks ()
+  "Return the length of longest name of bookmarks and maildirs."
+  (cl-loop for b in (append (mu4e-bookmarks)
+                            (mu4e~maildirs-with-query))
+           maximize (length (plist-get b :name))))
 
 
 ;;; Indexing & Updating