app: improve a bit the restart-triggering change list.

When you change a settings in Preferences which requires a restart of
GIMP, the current list was pretty ugly. I am still not fond of the popup
but at least now the list formatting is slightly better:

- Use a unicode bullet point character in UTF-8 encoding (since
  gtk_message_dialog_new() expects UTF-8 input) on each list element,
  instead of just writing them without any kind of formatting.
- Use the "nick" of the changed config, not the name. The "name" is an
  internal string which is really not meant to be viewed by people,
  while the "nick" is more like a short human-readable name or title,
  exactly what we want to show here.

app/dialogs/preferences-dialog.c

@@ -388,7 +388,10 @@ prefs_response (GtkWidget *widget,
               {
                 GParamSpec *param_spec = list->data;
 
-                g_string_append_printf (string, "%s\n", param_spec->name);
+                /* The first 3 bytes are the bullet unicode character
+                 * for doing a list (U+2022).
+                 */
+                g_string_append_printf (string, "\xe2\x80\xa2 %s\n", g_param_spec_get_nick (param_spec));
 
                 if (g_strcmp0 (param_spec->name, "language") == 0)
                   {