Fix spelling in comments and documentation using codespell

Apply the utility from https://github.com/codespell-project/codespell/
to fix spelling issues in the headers under both include and interface
directories and add a file with a couple of exceptions.

The exact command line used was:

    $ codespell -w -I misc/scripts/codespell.ignore -i 3 in*

interface/wx/dnd.h

@@ -146,7 +146,7 @@ public:
     /**
        Sets the default action for drag and drop.  Use wxDragMove or
        wxDragCopy to set default action to move or copy and use wxDragNone
-       (default) to set default action specified by initialization of draging
+       (default) to set default action specified by initialization of dragging
        (see wxDropSource::DoDragDrop())
     */
     void SetDefaultAction(wxDragResult action);