Switch to translating keystrokes using ToUnicodeEx rather than

ToAsciiEx, where possible.

This enables support for keys which generate Unicode characters that
aren't in the system code page, which seems to me like a perverse way
for Windows to have set up the system code page but apparently does
happen, e.g. (I'm told) U+0219 and U+021B on Romanian keyboards.

Patch mostly due to Andrei Damian-Fekete.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9942 cda61777-01e9-0310-a592-d414129be87e