Remove unused #include <Xw_Window.hxx>, causing trouble on OSX with Cocoa

Fixes issue #370.
Also remove #include <WNT_Window.hxx> on Windows

It is very unlikely that it is used, we will put it back if needed.
Merge pull request #371 from tpaviot/review/fix-build-V3d-Cocoa

Remove unused #include<Xw_Window.hxx> causing trouble on OSX with Cocoa