Use an appropriate icon automatically in wxMSW wxMessageDialog.

Use an appropriate icon automatically in wxMSW wxMessageDialog.

wxMessageBox() functions uses wxICON_QUESTION or wxICON_INFORMATION
automatically if no other icon style (including wxICON_NONE) is specified but
wxMessageDialog under MSW didn't do this, unlike under e.g. wxGTK.

Fix this by also determining the appropriate icon automatically in
wxMessageDialog itself in wxMSW.

Ideally we'd do this in all ports wxMessageDialog implementations and remove
the code doing it from wxMessageBox() but right now it's still needed for the
platforms other than wxMSW and wxGTK.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63683 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775