richedit: Implemented EM_STOPGROUPTYPING, fixed undo grouping (with tests)

[Dylan Smith]

EM_STOPGROUPTYPING simply ends the undo coalescing transaction.  The
remarks for this message on MSDN explains what events stops group
typing, which led me to adding the delete key to the actions that stop
group typing.

The tests that are included with this patch verify the correctness of
the changes.

This patch depends on a previous patch I submitted, which has an email
subject "[6/44] richedit: Implemented undo coalescing to group typing
events", and was submitted on June 17.
---
 dlls/riched20/editor.c       |   12 ++++++++++--
 dlls/riched20/tests/editor.c |   40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)
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