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/po/remove-potcdate.sin

@@ -0,0 +1,19 @@
+# Sed script that remove the POT-Creation-Date line in the header entry
+# from a POT file.
+#
+# The distinction between the first and the following occurrences of the
+# pattern is achieved by looking at the hold space.
+/^"POT-Creation-Date: .*"$/{
+x
+# Test if the hold space is empty.
+s/P/P/
+ta
+# Yes it was empty. First occurrence. Remove the line.
+g
+d
+bb
+:a
+# The hold space was nonempty. Following occurrences. Do nothing.
+x
+:b
+}