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/defs.h

@@ -98,7 +98,7 @@
      */
     #pragma warning(disable: 4127) /*  conditional expression is constant */
 
-    /* There are too many false positivies for this one, particularly when
+    /* There are too many false positives for this one, particularly when
        using templates like wxVector<T> */
     /* class 'foo' needs to have dll-interface to be used by clients of
        class 'bar'" */