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/testing.h

@@ -320,7 +320,7 @@ public:
 
     // Called to verify that all expectations were met. This cannot be done in
     // the destructor, because ReportFailure() may throw (either because it's
-    // overriden or because wx's assertions handling is, globally). And
+    // overridden or because wx's assertions handling is, globally). And
     // throwing from the destructor would introduce all sort of problems,
     // including messing up the order of errors in some cases.
     void CheckUnmetExpectations()