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/defs.h

@@ -1937,17 +1937,9 @@ enum wxStandardID
 /*  wxWindowID type                                                              */
 /*  ---------------------------------------------------------------------------- */
 
-/*
- * wxWindowID used to be just a typedef defined here, now it's a class, but we
- * still continue to define it here for compatibility, so that the code using
- * it continues to compile even if it includes just wx/defs.h.
- *
- * Notice that wx/windowid.h can only be included after wxID_XYZ definitions
- * (as it uses them).
- */
-#if defined(__cplusplus) && wxUSE_GUI
-    #include "wx/windowid.h"
-#endif
+/* Note that this is defined even in non-GUI code as the same type is also used
+   for e.g. timer IDs. */
+typedef int wxWindowID;
 
 /*  ---------------------------------------------------------------------------- */
 /*  other constants */

include/wx/menuitem.h

@@ -21,6 +21,8 @@
 
 #include "wx/object.h"  // base class
 
+#include "wx/windowid.h"
+
 // ----------------------------------------------------------------------------
 // forward declarations
 // ----------------------------------------------------------------------------

include/wx/utils.h

@@ -20,7 +20,6 @@
 #include "wx/filefn.h"
 #include "wx/hashmap.h"
 #include "wx/versioninfo.h"
-#include "wx/windowid.h"
 #include "wx/meta/implicitconversion.h"
 
 #if wxUSE_GUI

include/wx/window.h

@@ -27,6 +27,7 @@
 #include "wx/intl.h"
 
 #include "wx/validate.h"        // for wxDefaultValidator (always include it)
+#include "wx/windowid.h"
 
 #if wxUSE_PALETTE
     #include "wx/palette.h"

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

src/common/windowid.cpp

@@ -22,9 +22,7 @@
 #endif //WX_PRECOMP
 
 #include "wx/hashmap.h"
-
-// Not needed, included in defs.h
-// #include "wx/windowid.h"
+#include "wx/windowid.h"
 
 namespace
 {