Add PrivData() field to wxPrintData since MSW's printer

  Add PrivData() field to wxPrintData since MSW's printer
    sometimes seem to save data in an extra field. The
    original author of the patch chose to save it in char*
    so I kept that, although void* would probably be correct.
    The patch is against 2.4.2 and the code has been reshuffled
    since then so that the patch had to be redone entirely.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32443 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775