Small improvements to UI DPI scaling on Windows:

* Use USER_DEFAULT_SCREEN_DPI define from WinUser.h
* Change Win32 SDK target version to Windows Vista or greater
* Define WM_DPICHANGED as preprocessor definition as in WinUser.h
* Cull manual definitions of WM_MOUSEWHEEL and WHEEL_DELTA which are part of the Win32 SDK since Windows NT 4.0