1. Derive wxMessageDialogBase from wxDialog (why did we use multiple

   inheritance here?), moved more wxMessageDialog fields into the base class
2. Added functions to set the text of message box buttons (patch 1700393,
   currently only implemented for wxMac)
3. Added wxMessageDialog::SetMessage() and SetExtendedMessage() and
   implemented them for wxMac (part of the patch), wxGTK 2.4+ and in
   a generic way for everybody else
4. Allow changing the dialog attributes between creating and showing it,
   for this the real dialog creation is now done on in ShowModal() in all
   ports

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@46199 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775