1. wxDropTarget::OnData() returns wxDragResult now, not bool

1. wxDropTarget::OnData() returns wxDragResult now, not bool
2. fixed assert failure in wxMSW::wxListBox
3. wxFileHistory automatically deletes non existinf files from menu
4. wxDropTarget coordinates are client (and not screen) under MSW too
5. wxConvertBitmapToDib and vice versa seem to work!
6. client data field if filled by wxListBox and wxChoice and it's also deleted
   (this just wasn't done before!)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@4153 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775