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/dynload.h

@@ -39,7 +39,7 @@ typedef wxDLManifest wxDLImports;
 //       wxDL is not virtual and we deliberately hide some of it's
 //       methods here.
 //
-//       Unless you know exacty why you need to, you probably shouldn't
+//       Unless you know exactly why you need to, you probably shouldn't
 //       instantiate this class directly anyway, use wxPluginManager
 //       instead.