Simplify wxWindowID definition

Define it in wx/defs.h directly instead of conditionally including
wx/windowid.h from there just in order to get this type definition.

This has several advantages:

- wxWindowIDRef is not defined in (maybe) non-GUI code including
  wx/defs.h, as it should be the case for this class defined in
  core library only.
- wx/windowid.h becomes a normal header, including wx/defs.h as
  (almost) all the other ones instead of being exceptional.
- wx/windowid.h doesn't need to be included by wx/utils.h at all
  just to get wxWindowID definition.

Closes https://github.com/wxWidgets/wxWidgets/pull/1850

include/wx/windowid.h

@@ -10,9 +10,7 @@
 #ifndef _WX_WINDOWID_H_
 #define _WX_WINDOWID_H_
 
-// NB: do not include defs.h as we are included from it
-
-typedef int wxWindowID;
+#include "wx/defs.h"
 
 // ----------------------------------------------------------------------------
 // wxWindowIDRef: reference counted id value