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

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
 {