Turn the Windows PuTTY main window into a Unicode window.

Turn the Windows PuTTY main window into a Unicode window.

This causes WM_CHAR messages sent to us to have a wParam containing a
16-bit value encoded in UTF-16, rather than an 8-bit value encoded in
the system code page.

As far as I can tell, there aren't many other knock-on effects - e.g.
you can still interact with the window using ordinary char-based API
functions such as SetWindowText, and the Windows API will do the
necessary conversions behind the scenes. However, even so, I'm half
expecting some sort of unforeseen bug to show up as a result of this.