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
a2a3897124