riched20: Implement EM_LIMITTEXT and some simple tests.

[Matt Finnicum]

This just implements EM_LIMITTEXT and some simple tests. Since
EM_EXLIMITTEXT was already implemented, I didn't have to modify any
other functions to use the text limiting - I just had to have this
message set the nTextLimit variable properly. Note that MSDN claims
lots of nonsense about this message setting the limit to -1 for
multiline controls, but as the tests show - that's just not true.
Since it's an old message, I'm just assuming that behavior no longer
exists or matters.

Changelog:
riched20: Implement EM_LIMITTEXT and some simple tests.

--Matt Finnicum

 dlls/riched20/editor.c       |   11 +++++++++--
 dlls/riched20/tests/editor.c |   41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)