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 */