search: skip combining diacritical marks in search operations

https://bugzilla.gnome.org/show_bug.cgi?id=648587

src/shell-util.h

@@ -20,6 +20,8 @@ int      shell_util_get_week_start             (void);
 
 char    *shell_util_normalize_and_casefold     (const char       *str);
 
+char    *shell_util_normalize_casefold_and_unaccent (const char  *str);
+
 char    *shell_util_format_date                (const char       *format,
                                                 gint64            time_ms);