The files defining classes processing events on file descriptor only need this
class and not wxFDIODispatcher itself so reduce build dependencies by extracting
wxFDIOHandler in a separate header which they can include instead of the
entire fdiodispatcher.h.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61686 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
