Add WXWNDPROC typedef and use it in wxMSW instead of WXFARPROC

WNDPROC and FARPROC are not the same thing in MSW and it's wrong to use
WXFARPROC as the type of different window procedures we use.

Introduce WXWNDPROC which is more clear and correct and use it instead.

Also get rid of a few casts which are not necessary any longer.

include/wx/defs.h

@@ -2845,11 +2845,18 @@ typedef wxW64 long         WXLPARAM;
 typedef wxW64 long         WXLRESULT;
 #endif
 
+/*
+   This is defined for compatibility only, it's not really the same thing as
+   FARPROC.
+ */
 #if defined(__GNUWIN32__)
 typedef int             (*WXFARPROC)();
 #else
 typedef int             (__stdcall *WXFARPROC)();
 #endif
+
+typedef WXLRESULT (wxSTDCALL *WXWNDPROC)(WXHWND, WXUINT, WXWPARAM, WXLPARAM);
+
 #endif /*  __WIN32__ */