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*

include/wx/xtistrm.h

@@ -233,7 +233,7 @@ public:
     // The return value is the root object ID, which can
     // then be used to ask the depersister about that object
     // if there was a problem you will get back wxInvalidObjectID and the current
-    // error log will carry the problems encoutered
+    // error log will carry the problems encountered
     virtual int ReadObject( const wxString &name, wxObjectReaderCallback *readercallback ) = 0;
 
 private: