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/private/window.h

@@ -76,7 +76,7 @@ public:
         }
 
         // Ensure that we return true the first time we're called,
-        // asuming that the value will always be set to a non-default value.
+        // assuming that the value will always be set to a non-default value.
         return m_value == T();
     }