Use "MSW" and "Windows" more accurately to avoid confusion.

"MSW" is used as the name of toolkit using the standard Windows UI while
"Windows" is the name of the OS under which wxGTK can be now used too (in
addition to wxMSW).

This terminology is not perfect but at least let's keep it unambiguous.

Closes #15328.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74502 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775