Improve AltGr detection even further on Windows so we can detect the
case when Ctrl and AltGr are pressed together.

Ctrl+Pause sends break which results in a different VKEY on Windows.
Make sure we map that properly to the equivalent keysym.

This key requires a bit extra care on Windows. We get wildly
different messages depending on if Alt is pressed or not. The symbol
chosen now follows the standard behaviour on X11.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4449 3789f03b-4d11-0410-bbf8-ca57d06f2519