tag editor: add modified_item_prefix to see modified items

doc/config

@@ -226,6 +226,8 @@
 #
 #selected_item_suffix = "$9"
 #
+#modified_item_prefix = "$2> $9"
+#
 ## colors are not supported for below variable
 #
 #song_window_title_format = "{%a - }{%t}|{%f}"

doc/ncmpcpp.1

@@ -159,6 +159,9 @@ Prefix for selected items.
 .B selected_item_suffix = TEXT
 Suffix for selected items.
 .TP 
+.B modified_item_prefix = TEXT
+Prefix for modified item (tag editor).
+.TP 
 .B alternative_header_first_line_format = TEXT
 Now playing song format for the first line in alternative user interface header window.
 .TP