instantiates as well as forward declares, which gives us multiple definition
link errors on some systems. So trying WXDLLIMPEXP_FWD_XRC on the declaration
of m_handlers instead.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48775 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775