Rename wxCharCode{MSWToWX,WXToMSW}() and move them to a separate header.

Rename wxCharCode{MSWToWX,WXToMSW}() and move them to a separate header.

These functions were confusingly named as they work with MSW (virtual) key
codes and not character codes, rename them to better indicate what they do.
They also don't need to be in wx/msw/window.h included by all wx code when
they are only really needed in a couple of files, so move them to a private
header.

No changes in behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65523 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775