Add user32_vista.dll to introduce new NT6+ User32 features without changing the existing User32.dll when compiled as NT5.x. Also implements a stub for GetDpiForWindow(). The GetDpiForWindow() function will be required to Wine-sync common controls to modern Wine versions.
Changes:
Expose GetDpiForWindow() function and USER_DEFAULT_SCREEN_DPI to appropriate versions in winuser.h
Introduce a basic user32_vista library that can be expanded as needed.
