mu4e-mark: Add ability to use mu4e-use-fancy-chars for marking

Right now we have this for showing the status of messages, e.g. whether
it has an attachment etc. But not for the "d", "D" etc. in the leftmost
column of the headers view.

This adds support for that, while bending over backwards to ensure that
anyone who's customized this in the past won't have their customizations
broken, i.e. like `mu4e-headers-trashed-mark` we can set this to a cons
cell of basic/fancy characters, but we also continue to support this
just being a string for existing users.

The next patch in this series adds a couple of non-ASCII characters to
be used for the trash / delete mark.

mu4e/mu4e-mark.el

@@ -124,7 +124,13 @@ is either a headers or view buffer."
   "The list of all the possible marks.
 This is an alist mapping mark symbols to their properties.  The
 properties are:
-  :char (string)  The character to display in the headers view
+  :char (string) or (basic . fancy) The character to display in
+    the headers view. Either a single-character string, or a
+    dotted-pair cons cell where the second item will be used if
+    `mu4e-use-fancy-chars' is `t' or `marks', otherwise we'll use
+    the first one. It can also be a plain string for backwards
+    compatibility since we didn't always support
+    `mu4e-use-fancy-chars' here.
   :prompt (string) The prompt to use when asking for marks (used for
      example when marking a whole thread)
   :ask-target (function returning a string) Get the target.  This
@@ -235,7 +241,14 @@ The following marks are available, and the corresponding props:
 	  ;; target (the target folder) the other ones get a pseudo "target", as
 	  ;; info for the user.
 	  (markdesc (cdr (or (assq mark mu4e-marks) (mu4e-error "Invalid mark %S" mark))))
-	  (markkar (plist-get markdesc :char))
+	  (get-markkar
+	   (lambda (char)
+	     (if (listp char)
+	         (if (or (eq mu4e-use-fancy-chars t)
+	                 (eq mu4e-use-fancy-chars 'marks))
+	             (cdr char) (car char))
+	       char)))
+	  (markkar (funcall get-markkar (plist-get markdesc :char)))
 	  (target (mu4e~mark-get-dyn-target mark target))
 	  (show-fct (plist-get markdesc :show-target))
 	  (shown-target (if show-fct