Added support for mouse-wheel events on Windows.

To be used in the mouse callback like this:
    if (CV_EVENT_MOUSEWHEEL == CV_GET_MOUSEWHEEL_EVENT(event))
    {
       int delta= CV_GET_WHEEL_DELTA(event);
       // use delta...
    }

modules/highgui/include/opencv2/highgui/highgui_c.h

@@ -170,7 +170,9 @@ enum
     CV_EVENT_MBUTTONUP      =6,
     CV_EVENT_LBUTTONDBLCLK  =7,
     CV_EVENT_RBUTTONDBLCLK  =8,
-    CV_EVENT_MBUTTONDBLCLK  =9
+    CV_EVENT_MBUTTONDBLCLK  =9,
+    CV_EVENT_MOUSEWHEEL     =10,
+    CV_EVENT_MOUSEHWHEEL    =11
 };
 
 enum
@@ -183,6 +185,11 @@ enum
     CV_EVENT_FLAG_ALTKEY    =32
 };
 
+
+#define CV_GET_WHEEL_DELTA(event) ((short)((event >> 16) & 0xffff)) // upper 16 bits
+#define CV_GET_MOUSEWHEEL_EVENT(event) (event & 0xffff) // lower 16 bits
+
+
 typedef void (CV_CDECL *CvMouseCallback )(int event, int x, int y, int flags, void* param);
 
 /* assign callback for mouse events */