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/TODO

@@ -0,0 +1,9 @@
+Add --include option (opposite of --exclude).
+
+Look into sdiff improvement here:
+http://www.pkix.net/~chuck/sdiff2.diff
+
+Propagate stderr from subprocess so that diff3 does
+a better job of explaining _why_:
+  > $ diff3 /bin/sh /bin/false /bin/mv
+  > diff3: subsidiary program 'diff' failed (exit status 2)