wxContribItems duplicate block in header files

[Wkerry]

In allot of the wxContribItems header files the following block is duplicated in the header files that seems redundant if the code was changed to use the wxwidgets wx/dlimpexp.h header file:

#ifdef __WXMSW__
    #ifndef DLLEXPORT
        #define DLLEXPORT __declspec (dllexport)
    #endif
#else
    #define DLLEXPORT
#endif

The wx/dlimpexp.h has support for WXEXPORT and WXIMPORT for all of the supported platforms and makes the code easier to read if it was changed to use these instead of the DLLEXPORT in the block above.