Set the data size in wxEVT_COMMAND_DATAVIEW_ITEM_DROP_POSSIBLE in wxGTK.

Allow wxEVT_COMMAND_DATAVIEW_ITEM_DROP_POSSIBLE handler to examine the data
being dragged by setting not only the pointer to it but also its size, as
otherwise it's impossible to do anything with it.

See #12583.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71323 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/dataview.cpp

@@ -3594,6 +3594,7 @@ wxDataViewCtrlInternal::row_drop_possible(GtkTreeDragDest *WXUNUSED(drag_dest),
     event.SetItem( item );
     event.SetModel( m_wx_model );
     event.SetDataFormat(gtk_selection_data_get_target(selection_data));
+    event.SetDataSize(gtk_selection_data_get_length(selection_data));
     if (!m_owner->HandleWindowEvent( event ))
         return FALSE;