Implement LBS_COMBOBOX, and make use of it.
Better separation between the Listbox and Combobox.
Have a single set of wndprocs, just like Windows.

diff --git a/include/winuser.h b/include/winuser.h
index 700c4ef..9a242c2 100644
--- a/include/winuser.h
+++ b/include/winuser.h
@@ -2164,6 +2164,7 @@
 #define LBS_DISABLENOSCROLL      0x1000
 #define LBS_NODATA               0x2000
 #define LBS_NOSEL                0x4000
+#define LBS_COMBOBOX             0x8000
 #define LBS_STANDARD  (LBS_NOTIFY | LBS_SORT | WS_VSCROLL | WS_BORDER)
 
 /* Listbox messages */