Add mouse wheel handling.

This includes windowsx.h in order to use GET_X_LPARAM and GET_Y_LPARAM, which provide the correct mouse wheel coordinates in all cases, including multiple monitors in any orientation. For WM_MOUSEWHEEL, LOWORD and HIWORD work correctly on monitors with positive screen coordinates, but give incorrect results on monitors with negative screen coordinates.

For reference, see: http://msdn.microsoft.com/en-us/library/windows/desktop/ms645617(v=vs.85).aspx