Reorganize the code by putting OLE-dependent code into the blocks
controlled by wxUSE_OLE and by sharing remaining code (like implementation
of wxDataFormat, wxBitmapDataObject, wxFileDataObject, etc.) to allow
building wxDataObject and its specializations also without OLE support.
Since wxDataObject no longer requires OLE support, corresponding check in
checkconf.h can be removed.
Thanks to this additional flexibility, it is possible to use wxClipboard
whether OLE support (wxUSE_OLE) is enabled or not, either with OLE-based
wxDataObject (OLE clipboard) or with wxDataObject decoupled from OLE (Win
clipboard API).
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