D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/lib/ref-add.sin

@@ -0,0 +1,29 @@
+# Add this package to a list of references stored in a text file.
+#
+#   Copyright (C) 2000, 2009-2017 Free Software Foundation, Inc.
+#
+#   This program is free software; you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation; either version 3, or (at your option)
+#   any later version.
+#
+#   This program is distributed in the hope that it will be useful,
+#   but WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#   GNU General Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program; if not, see <http://www.gnu.org/licenses/>.
+#
+# Written by Bruno Haible <haible@clisp.cons.org>.
+#
+/^# Packages using this file: / {
+  s/# Packages using this file://
+  ta
+  :a
+  s/ @PACKAGE@ / @PACKAGE@ /
+  tb
+  s/ $/ @PACKAGE@ /
+  :b
+  s/^/# Packages using this file:/
+}