Implemented the WM_SETICON and WM_GETICON messages.

diff --git a/include/winuser.h b/include/winuser.h
index 9bc8d99..d1b3d60 100644
--- a/include/winuser.h
+++ b/include/winuser.h
@@ -938,6 +938,10 @@
 #define WA_ACTIVE               1
 #define WA_CLICKACTIVE          2
 
+/* WM_GETICON/WM_SETICON params values */
+#define ICON_SMALL              0
+#define ICON_BIG                1
+
   /* WM_NCCALCSIZE parameter structure */
 typedef struct
 {