Intern strings for use as plist keys

`plist-get` and `plist-put` don't work properly otherwise.

org-generic-id.el

@@ -230,7 +230,7 @@ diagnostic and progress messages."
                   ((file
                     (car-safe
                      (org-generic-id-files-modified-since-modtime
-                      (plist-get org-generic-id--last-update-id-time id-prop)
+                      (org-generic-id--last-update-id-time-get id-prop)
                       (list file)
                       org-generic-id--files))))
                 (unless silent
@@ -268,7 +268,7 @@ diagnostic and progress messages."
     ;; Save the new locations and reload to regenerate variables.
     (org-generic-id-locations-save)
     (org-generic-id-locations-load)
-    (plist-put org-generic-id--last-update-id-time id-prop (current-time))
+    (org-generic-id--last-update-id-time-put id-prop (current-time))
     (when (and (not silent) (> ndup 0))
       (warn
        "WARNING: %d duplicate :%s: properties found, check *Messages* buffer"
@@ -496,6 +496,20 @@ FILE-TO-BUF, whose format is documented at ‘org-generic-id--files’."
          file-to-buf file buf)
         buf))))
 
+(defun org-generic-id--last-update-id-time-get (id-prop)
+  "Get last update ID time for ID-PROP.
+ID-PROP is a string.  This function converts ID-PROP to a symbol in order to
+query ‘org-generic-id--last-update-id-time' using ‘plist-get’."
+  (plist-get org-generic-id--last-update-id-time (intern id-prop)))
+
+(defun org-generic-id--last-update-id-time-put (id-prop time)
+  "Set last update ID time for ID-PROP to TIME.
+ID-PROP is a string; TIME is an Emacs time value as returned by ‘encode-time’.
+This function converts ID-PROP to a symbol in order to query
+‘org-generic-id--last-update-id-time' using ‘plist-put’."
+  (plist-put org-generic-id--last-update-id-time
+             (intern id-prop) time))
+
 (defun org-generic-id--files-find-file-hook ()
   "Update ‘org-generic-id--files’ after a file is loaded."
   (org-generic-id--files-buffer-hook-impl