pdTextBox: I *think* all Unicode input methods now work

Methods I have tested:

- Regular text entry
- Alt+ keycodes
- OSK with non-Latin locales (including Kazakh and other known
problematic input methods, e.g.
http://www.vbforums.com/showthread.php?779293-Unicode-Textbox&p=4782515&viewfull=1#post4782515).
This is a big win, as all other open-source CreateWindowEx
implementations I've seen don't handle this correctly.
- Dead keys, via US International keyboard and `+a, etc.  Another big
win.
- Microsoft on-screen IME pad (traditional Chinese - Taiwan locale)
- Pasting Unicode text

I am *really* hoping others can test this with actual writing pads
(which I do not have) and let me know what happens.  There's no reason
those shouldn't work as well, but I know better than to curse myself by
making promises.

With this nasty work out of the way, I can finally start working on
making the rest of the control work correctly.