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/xml/xml.h

@@ -171,7 +171,7 @@ public:
     void SetAttributes(wxXmlAttribute *attr) { m_attrs = attr; }
     virtual void AddAttribute(wxXmlAttribute *attr);
 
-    // If true, don't do encoding conversion to improve efficiency - node content is ACII text
+    // If true, don't do encoding conversion to improve efficiency - node content is ASCII text
     bool GetNoConversion() const { return m_noConversion; }
     void SetNoConversion(bool noconversion) { m_noConversion = noconversion; }