Patch from Yoshida Masato to fill in the missing pieces of Windows

Patch from Yoshida Masato to fill in the missing pieces of Windows
UTF-16 support. High Unicode characters in the terminal are now
converted back into surrogates during copy and draw operations, and
the Windows drawing code takes account of that when splitting up the
UTF-16 string for display. Meanwhile, accidental uses of wchar_t have
been replaced with 32-bit integers in parts of the cross-platform code
which were expecting not to have to deal with UTF-16.

git-svn-id: http://svn.tartarus.org/sgt/putty@9409 cda61777-01e9-0310-a592-d414129be87e